Terminologies used in Epidemiological Studies

• Public health: Public health is defined as the wellbeing of the general public. It is the field of medical science that maintain a surveillance and monitoring function over the general population by probing the water supply, food and other materials that enters the human body in a view to  keep them safe from pathogens and also to prevent the spread of infectious diseases through them in a defined human population. Public health is a healthcare service that is provided to the general public by the local, state or federal government of any nation including those other healthcare, sanitation, cleaning and environmental services rendered by organizations such as the World Health Organization (WHO), Center for Disease Control and Prevention (CDC), United Nations (UN) and UNICEF. 

• Health determinants: Health determinants are those fundamental or primary environmental factors including social, cultural, religious and economic factors that are directly or indirectly responsible for the evolving of disease or better health of a people. 

• Population: A population is a collection of people or organisms of the same type.

• Population in epidemiological terms can also be used synonymously as community. It also stands for society, the general public or a group of people or animals being understudied for a particular disease at a given time and in a particular geographic site.   

• Bias: Bias is a systematic error that results in an unfounded or incorrect estimate of the measure of relationship between the cause and effect of a disease. It can result in virtually all epidemiological study (either from the investigator or subjects of the study) and when it happens, bias can result to the masking of a relationship between cause and effect of a disease where one actually exist.

• Risk factors: A risk factor is any characteristic, behavior, environmental exposure, or biological condition associated with an increased likelihood of developing a disease or adverse health outcome. Examples include smoking, obesity, diet, alcohol use, hypertension, and exposure to pollutants. Identifying risk factors is central to epidemiological research because it helps explain disease causation and supports prevention efforts. Risk factors may be modifiable or non-modifiable. Epidemiologists use statistical analyses to determine the strength of associations between risk factors and diseases, providing evidence for public health interventions. Risk factors are environmental issues or personal tendencies that increase one’s probability or chance to acquire a disease or an infection in a defined population. Risk factors can be used to envisage future outbreak of a disease/infection.

• Nosocomial infections: nosocomial infections are diseases that are hospital-associated and develop inside a clinic facility, and are acquired after a prior stay in the hospital environment.

• Epidemiology: Epidemiology is the scientific study of the distribution, determinants, and control of health-related events in populations. It seeks to understand who becomes ill, where diseases occur, when they occur, and why they occur. Epidemiologists use systematic methods to identify risk factors, disease patterns, and preventive measures. The discipline provides evidence for public health policies, disease surveillance, outbreak investigations, and healthcare planning. It serves as the foundation for understanding population health and developing interventions that reduce disease burden and improve overall health outcomes.

• Population at Risk: A population at risk refers to the group of individuals who have the potential to develop a particular disease or health outcome during a specified period. Individuals already affected by the disease are usually excluded from this population. Defining the population at risk accurately is essential for calculating epidemiological measures such as incidence rates and relative risks. Researchers use this concept to identify vulnerable groups, allocate resources effectively, and evaluate disease prevention strategies. The population at risk may vary depending on demographic, environmental, occupational, or genetic characteristics.

• Quarantine: Quarantine is the medical practice of limiting the free movement of patients that have been screened and confirmed to have a highly communicable disease (e.g. tuberculosis which is caused by a species of Mycobacterium). The sole reason for quarantining an individual with an infectious disease is to prevent the spread of the pathogen and the disease to susceptible members of the public, while the infected person is carefully treated and supervised to ensure that the disease is totally eradicated. Quarantine can also be used to describe the isolation of an immigrant or an animal in order to look out for an infectious disease in them prior to their entering a particular country. The diseases that require quarantine by international conformity include: typhoid fever, yellow fever, small pox, plague, relapsing fever and cholera.

• Surveillance: Surveillance which means inspection or observation is an epidemiological technique of observing, recognizing and reporting a disease outbreak to the appropriate authority (e.g. state or local ministry of health, WHO, the CDC and other public health agencies) as they occur in order to forestall their sporadic spread to an uninfected community.  

• Outbreak: Outbreak is a medical term which is used to describe the sudden and unprecedented incidence of a particular disease in a specific community over a short time period. 

• Validity: This is the expression of the degree to which an epidemiological study or test is capable of measuring what it is intended to measure. A test result is valid when the result of such a study corresponds to the truth of the study. It is the lack of bias and confounding factors in a study. 

• Convalescent period of disease: Convalescent period which can also be referred to as recuperative period is the period of recovery from a particular infection in which a previously ill patient regains his or her normal body functions and returns back to normal life free of the disease.

• Convalescent carrier: A convalescent carrier is an individual who have just recovered from a particular disease but who still carry large amounts of the pathogen responsible for the infection.  

• Decline period of disease: Decline period of an infection is the phase of fast or slow reduction in the signs and symptoms of a disease, and which allows the sick individual to regain his or her normal wellbeing.

• Index: Index is the next estimate to a rate or the best available approximation to a true value. It is usually used when it becomes difficult to directly count the number of people at risk of acquiring an infection in a population.

• Proportion: Proportion is an expression of an event in which the numerator is always included in the denominator. It is always expressed as a percentage.

• Ratio: Ratio is the relationship between a numerator and a denominator. It is statistically expressed as: A:B; where A = numerator, B = denominator 

• Rate: Rate measures the possibility or probability of occurrence of some particular episode(e.g. disease or infection) in a given community/population. The rate of an event is usually expressed statistically as: A/B X C; where A = number of event at a time period, B = number of individuals exposed to the event, C = a constant or round number (e.g. 100, 1,000, 10,000 et cetera). 

• Exposure: Exposure refers to contact with a factor that may influence health outcomes, such as pathogens, chemicals, radiation, lifestyle behaviors, or environmental conditions. Epidemiological studies examine exposures to determine whether they contribute to disease development. Exposure assessment involves measuring frequency, duration, intensity, and timing. Accurate exposure measurement is critical for establishing causal relationships between risk factors and health outcomes. Researchers often compare exposed and unexposed groups to evaluate disease risk and identify preventive measures that can reduce harmful exposures.

• Outcome: An outcome is the health event, disease, condition, or status being studied in epidemiological research. Outcomes may include infections, chronic diseases, recovery, disability, or death. Researchers define outcomes clearly to ensure consistent measurement and interpretation. The relationship between exposures and outcomes forms the basis of epidemiological investigations. Outcomes can be measured through clinical diagnoses, laboratory tests, surveys, or health records. Accurate outcome assessment improves study validity and helps determine the effectiveness of interventions or the impact of risk factors.

• Mortality: Mortality is defined as the occurrence of death from a particular disease in a given population. Mortality refers to the occurrence of death within a population and is a key indicator of population health. Mortality rates measure the frequency of deaths over a specified period and can be categorized as crude, age-specific, disease-specific, or cause-specific mortality. Epidemiologists analyze mortality data to identify leading causes of death, evaluate health interventions, and assess population health status. Mortality statistics are essential for healthcare planning and policy development. Trends in mortality often reflect changes in disease patterns, healthcare access, and socioeconomic conditions.

• Morbidity: Morbidity is defined as the occurrence of infection (without death) in a particular community. It is used to describe nonfatal and fatal diseases in a population. Morbidity is the state of being unhealthy or diseased following the invasion of a pathogen into the body. Morbidity describes the state of illness, disease, or poor health within a population. It encompasses both the occurrence and burden of diseases, injuries, and disabilities. Morbidity measures help quantify the impact of health conditions on individuals and communities. Epidemiologists use morbidity data to assess healthcare needs, evaluate disease prevention programs, and monitor public health trends. Morbidity can be measured through incidence, prevalence, hospitalization rates, or disability-adjusted indicators. Understanding morbidity patterns supports informed decision-making in healthcare policy and disease management.

• Prevalence: Prevalence is used to describe the total burden of disease in a community. It is used to show the old and new infections that occur in a population at a given time. Prevalence shows the number of individuals in a population who has the disease of interest at a given time period. Prevalence is the total number of existing cases of a disease or health condition within a population at a specific time or during a particular period. Unlike incidence, prevalence includes both new and old cases. It provides a snapshot of disease burden and is useful for healthcare planning and resource allocation. High prevalence may result from high incidence, prolonged disease duration, or both. Epidemiologists often use prevalence to assess the impact of chronic diseases, estimate healthcare needs, and identify populations requiring targeted interventions.

• Incidence: Incidence is used to describe the number of new cases of a disease that is reported in a given community.   

• Confounding factors: Confounding factors (or variables) are confusing factors in arise in an epidemiological study and, which are directly or indirectly related to one or more of the variables already defined in the study. They may show a false association between the study variables and may sometimes conceal an actual relationship between the cause and effect of a disease in a given population. Bias may result in such an epidemiological study if confounding factors are not taken into consideration during the design of the study. Confounding factors can be controlled by effective randomization, matching of subjects and restriction or stratification. 

• Health indicators: These are variables or factors that can be measured directly by public health workers and other health policy makers in order to establish the exact state of health of members of a given population/community. Typical examples of such indicators include life expectancy, education, and economic development which can be used to determine the Human Development Index (HDI) of a people or nation.   

• Endemic: Endemic refers to the constant presence or usual occurrence of a disease within a specific geographic area or population. Endemic diseases maintain relatively stable levels over time without requiring external introduction. Examples include malaria in certain tropical regions. Understanding endemic patterns helps public health officials allocate resources, monitor disease trends, and design control strategies. Changes in endemic levels may signal emerging health problems or successful interventions. Endemic diseases often reflect long-term interactions among pathogens, hosts, and environmental factors.

• Epidemic: An epidemic occurs when the number of disease cases exceeds the expected level within a specific population or geographic area over a particular period. Epidemics may result from infectious diseases, environmental exposures, or behavioral factors. Epidemiologists investigate epidemics to identify causes, transmission pathways, and effective control measures. Rapid detection and response are essential to minimize health impacts. Epidemic investigations often involve surveillance, case finding, contact tracing, and statistical analyses to understand disease spread and implement preventive interventions.

• Pandemic: A pandemic is an epidemic that spreads across multiple countries or continents, affecting a large number of people. Pandemics typically involve infectious diseases capable of sustained human-to-human transmission. Historical examples include influenza pandemics and COVID-19. Pandemics present significant challenges for healthcare systems, economies, and societies. Epidemiologists play a critical role in monitoring transmission patterns, evaluating interventions, and informing policy decisions. Effective pandemic management requires international collaboration, surveillance systems, vaccination programs, and public health preparedness measures.

• Endemic disease: Endemic diseases are infectious diseases that are constantly present at very low frequency in a particular population. Such a disease affects only relatively low members (or few persons) of the population at a given time period. The low incidence of such diseases in a population may be due to different reasons including the high immunity of individuals in the community or the low virulence of the disease agent. Examples of endemic diseases include common cold, leprosy and tuberculosis (TB). An endemic is a disease that maintains a stable and a low-level occurrence within a particular population over a given period of time. Endemic diseases are always present in a community in a particular smaller geographical area, and they affect a few members of the population.

• Epidemic disease: Epidemic diseases are infectious diseases that increase rapidly in frequency above the tolerable or predictable level in a particular population over a given period of time. It is a disease that affects a reasonable high number of people in a community at a given time. Epidemics are usually characterized by a rapid onset of clinical signs and symptoms of a disease that affects susceptible hosts at the same and, they involve a sizeable proportion of individuals within a population for a limited time period. The outbreak of a disease (infectious or non-infectious) in a particular population whose incidence obviously surpasses the incidence of a similar disease as obtainable in another population is termed an epidemic. Some examples of epidemic diseases include AIDS, poliomyelitis, and influenza. Epidemic diseases are those infections that show a very high prevalence in a wider national geographic area and, such diseases usually emerged from an endemic.  

• Pandemic disease: Pandemic diseases are epidemic diseases that have a widespread reach, affecting more than one country. It is an increase in the occurrence of a series of epidemics within a large population of people that cut across continents of the world. Pandemics are worldwide epidemics. Some examples of pandemic diseases include coronavirus disease 2019 (COVID-19), AIDS and influenza outbreak.

• Sporadic disease: Sporadic diseases are those diseases that occur irregularly and at a periodic or random interval amongst susceptible members of a population. Such diseases do not occur frequently but rather occasionally in a given community, and they show immediate clinical signs and symptoms amongst affected susceptible hosts in a population. Sporadic diseases are usually uncommon, and when they occur in a population, it affects only a relatively few individuals in that community. Examples of sporadic diseases include: scarlet fever, diphtheria and typhoid fever.

• Index case of a disease: The index case of a disease is the first case of a disease recorded in an epidemic in a particular population.

• Hyper-endemic disease: Hyper-endemic diseases are seasonal diseases that often increase in occurrencefrom an endemic status to a near epidemic status in a particular population. Example of such a disease is common cold which usually rises gradually in occurrence during the cold seasons. 

• Zoonosis: Zoonoses are infectious diseases which can be contracted by human beings and in which the disease-causing agent responsible for the disease is generally maintained in an animal population. They are infections/diseases that are transmitted from animal population to human population through direct or indirect contacts with these animals. Examples of zoonotic diseases include: anthrax, avian influenza and rabies. 

• Pathogen: Pathogen is any microorganism which by direct contact and invasion causes disease in another living organism (e.g. animals and human beings). Pathogens are disease-causing microorganisms and they include bacteria, virus, protozoa and fungi.

• Infection: Infection is the initial entry of disease-causing microorganism or pathogen into a susceptible host. The pathogen becomes established within the host after a successful entry. It should be noted that infection does not necessarily lead to a disease until the pathogen attacks one or more organs of the host via the release of toxigenic, pathogenic and virulent substances.

• Pathogenicity: Pathogenicity is simply defined as the ability of a pathogen to cause a disease. It is the percentage of individuals in a population who develop clinical disease without necessarily becoming severely sick.

• Virulence: Virulence is the level of pathogenicity generated by a disease-causing organism. It is the percentage of individuals with proven clinical infection, and who eventually becomes severely unwell by the disease. Virulence is also referred to as the intensity of pathogenicity of an organism as shown in the case fatality rate of an infection or disease.

• Opportunistic pathogen: Opportunistic pathogens are microorganisms that causes disease in the absence of normal host resistance i.e. when the immune system of a host is weakened or compromised by a disease (e.g. AIDS). They are free-living microorganisms which can occur as part of the body’s normal microflora or are found in the environment, and which may assume a pathogenic role in an individual under some certain conditions such as when the normal disease defense mechanism of the host (i.e. the immune system) has been impaired. 

• Infectious disease: Infectious diseases are diseases which occur in a host when there is a change in its normal state of health in which part or all of its normal body functions can no longer carry on. Such diseases are generally termed communicable diseases, and they can be transferred from an infected individual to uninfected susceptible host either directly or indirectly. Examples include measles, AIDS, hepatitis, syphilis, influenza.

• Non-infectious disease: Non-infectious diseases are those diseases that cannot be transmitted naturally from one individual to another either directly or indirectly. Some non-infectious (non-communicable) diseases are not caused by pathogens and this includes vitamin deficiency diseases, diabetes, cancer and some inherited birth and genetic defects. Others are caused by pathogens and are not transferred from an infected host to an uninfected individual, and example is tetanus caused by Clostridium tetani. Other non-infectious diseases include hypertension and diabetes. 

• Surveillance: Surveillance is the continuous and systematic collection, analysis, interpretation, and dissemination of health data for public health action. It enables early detection of disease outbreaks, monitoring of health trends, and evaluation of interventions. Surveillance systems may be passive, active, sentinel, or syndromic. Epidemiologists use surveillance data to identify emerging threats, assess disease burden, and guide public health responses. Effective surveillance provides timely and accurate information that supports evidence-based decision-making and improves disease prevention and control efforts.

• Case Definition: A case definition is a standardized set of criteria used to determine whether an individual should be classified as having a particular disease or health condition. It may include clinical symptoms, laboratory results, and epidemiological information. Consistent case definitions ensure comparability of data across studies and surveillance systems. Epidemiologists rely on case definitions during outbreak investigations and disease monitoring. Clear definitions reduce misclassification and improve the accuracy of incidence and prevalence estimates, supporting reliable public health assessments.

• Attack Rate: Attack rate is a measure commonly used during outbreaks to describe the proportion of individuals who become ill among a population at risk over a short period. Although termed a rate, it is technically a proportion. Attack rates help quantify the extent of disease spread and identify high-risk groups. Epidemiologists use attack rates to evaluate outbreaks in schools, workplaces, communities, and other settings. Comparing attack rates across groups can reveal potential sources of infection and guide targeted control measures.

• Cohort Study: A cohort study is an observational study design in which groups of individuals are followed over time to assess the occurrence of outcomes based on exposure status. Participants are classified as exposed or unexposed and monitored prospectively or retrospectively. Cohort studies are valuable for estimating incidence, relative risk, and temporal relationships between exposures and outcomes. They are particularly useful for studying rare exposures. Although often resource-intensive, cohort studies provide strong evidence for causal inference when conducted rigorously.

• Case-Control Study: A case-control study compares individuals with a disease or outcome (cases) to individuals without the disease (controls) to identify differences in previous exposures. This design is particularly efficient for studying rare diseases and conditions with long latency periods. Researchers calculate odds ratios to estimate associations between exposures and outcomes. Case-control studies are generally quicker and less expensive than cohort studies. However, they may be susceptible to recall bias and selection bias, requiring careful study design and interpretation.

• Cross-Sectional Study: A cross-sectional study examines exposures and outcomes simultaneously within a population at a single point in time. It provides information about prevalence and population characteristics but does not establish temporal relationships. Researchers commonly use cross-sectional studies for health surveys and needs assessments. These studies are relatively inexpensive and efficient for generating hypotheses. However, because exposure and outcome are measured at the same time, determining cause-and-effect relationships is often difficult. Despite limitations, cross-sectional studies are valuable tools in public health research.

• Randomized Controlled Trial: A randomized controlled trial (RCT) is an experimental study in which participants are randomly assigned to intervention or control groups. Randomization minimizes bias and balances confounding factors between groups. RCTs are considered the gold standard for evaluating treatment efficacy and preventive interventions. Researchers compare outcomes between groups to determine intervention effects. Well-designed RCTs provide strong evidence for causal relationships. Ethical considerations, costs, and logistical challenges can limit their feasibility, but they remain fundamental in clinical and epidemiological research.

• Symptoms: Symptoms are physically observed signs of a disease. They are changes that occur during the development of a disease in an individual, and they are experienced by an individual in the form of pain, fatigue, loss of appetite and body discomfort. 

• Syndromes: Syndromes are a set of clinical signs that are characteristics of a particular disease. They give impetus to the presence of a disease which warrants laboratory diagnosis. The syndrome of a disease is a combination of its signs and symptoms. 

• Signs: Signs are changes in a diseased individual which can be physically observed. Examples include rash on the body, rise in body temperature and boils. 

• Carrier: A carrier is an individual who harbours a particular microbe (responsible for a given disease) but shows no clinical signs or symptoms of the disease. Carriers of a disease-causing agent are important reservoirs of some infections. Such individuals can readily transmit the pathogen they are harbouring to susceptible persons. 

• Reservoir: Reservoirs are human or animal populations in which a disease-causing agent is preserved in a viable infective stage, and from which healthy individuals in a population may be infected. It is a site or carrier that harbours pathogens and also serves as sources of infection for susceptible individuals within a given population. 

• Infectivity period: Infectivity period is the period of an illness during which the infected host is infectious (i.e. disseminating the causative agent of the disease to susceptible hosts). Infectivity which also means infectiousness is the state of being communicable or infectious with a pathogen. Infectivity is the percentage of exposed individuals who eventually becomes infected by a particular disease-causing organism (pathogen).

• Source of a disease: The source of a disease is the site or material from which a disease-causing agent is immediately transferred to a susceptible human host either directly or indirectly.

• Active carrier: Active carriers are individuals who has an overt clinical infection (i.e. disease with obvious signs and symptoms), and who can be a source of transmission of the pathogen to other individuals in a given population. 

• Healthy carrier: A healthy carrier is a person who harbours a pathogen but is not sick or showing any clinical signs or symptoms associated with the disease caused by the agent. 

• Vectors: Vectors are living organisms (insects or animals) that help to transmit or carry disease-causing microorganisms from one individual to another. 

• Acute disease: Acute diseases are diseases that have a rapid beginning but which persist for a relatively short period of time. They are diseases which develop rapidly and can be dangerous. Such disease usually ends or terminates either in the recovery or death of the affected individual. Examples of acute diseases include Lassa fever.

• Chronic disease: A chronic disease is a long term disease that persists for a relatively lengthy period of time spanning into months and even years. Such diseases usually end in either the recovery or death of the affected host. Examples of such diseases include tuberculosis (TB), leprosy and AIDS.

• Notifiable disease: Notifiable diseases are those diseases that must be reported by law to the appropriate health authorities of a particular population so that appropriate action will be taken by the authorities to avert the spread of the disease. Such disease may affect human beings, plants or animals; and their suspected occurrence or happening in any part of a community must be reported without delay to the appropriate health authorities that will take action directed towards their proper containment and/or eradication in the affected population.

Normally, each state, local or federal government of a nation establishes by law those infections/diseases whose occurrence in a community must be reported by health care givers or doctors to the appropriate health authorities. Data emanating from such disease reports are used by public health personnel and other health care policy makers in the monitoring of the health status of a population to direct public health services appropriately. Examples of some notifiable diseases are coronavirus disease 2019 (COVID-19), Ebola, anthrax, AIDS, cholera, smallpox, Lassa fever, dengue fever, guinea worm disease, measles, poliomyelitis, tuberculosis (TB) and other enteric infections such as Salmonellosis amongst others.

The recent outbreak and spread of ebola virus disease (EVD) in some West African countries including Sierra Leone, Guinea, Liberia, Senegal and Nigeria in 2014 and even in the United States of America where a case was recorded is a typical example of the need to notify (i.e. alert) the public health authorities about the outbreak of a disease so that they can be quickly contained before they spread sporadically within the population. Some diseases such as the EVD are re-emerging with greater virulence; and it is important that the general public be sensitized about some of these infections and how they can alert the health authorities about any incidence in their community.    

• Antigenic drift: Antigenic drift is the minor change that occurs in the antigenic structure of a disease-causing agent that allows it to evade attack by the immune system of a host in the course of an infection. It is a mutational alteration in the surface proteins (which are target sites of antibodies) of pathogenic strains of microorganisms in such a way that antibodies that recognizes these antigenic sites do so less effectively or no longer recognizes them as it used to previously.

Such drift or subtle changes in the antigenic surfaces of pathogens are a setback for most vaccination exercises because some vaccines have a short lifespan and, thus new forms of such vaccines must be produced and administered in order to confer a better protection against the antigen it is directed to. Both antigenic drift and antigenic shift are significant in epidemiological surveys because they help epidemiologists to recognize a pandemic or an epidemic in a population with regards to the number of susceptible individuals to those that are actually exposed to the disease being investigated.

This is because whenever there is an antigenic drift or shift in a community, there is the likelihood of an increase in the number of susceptible individuals for the reason that the immune system of these persons has not yet been exposed to the mutant strains of the pathogens which resulted from a mutational change. In such populations, the morbidity rate will increase when the proportion of susceptible individuals is well above the allowed threshold density of susceptibles in the population.

• Antigenic shift: Antigenic shift is a major change in the antigenic structure of a pathogen that gives it the exceptional ability to be unrecognized by the immune system of a host. It is a mutational change in the antigenic structure of a microbe that allows it to change from one antigenic type to another and thus continue to remain unnoticed by the immune system mechanisms of a host. In antigenic shift, there is a reassortment or rearrangement of the genetic makeup (otherwise known as gene reassortment)of pathogenic strains of microbes, and this allows them to produce a distinct type of surface proteins that are quite different from the original surface proteins of the organism.

When antigenic drift or antigenic shift occurs in a population, it will be difficult or futile for a previous immunization exercise or immunity against a given infection to be beneficial and active against genetically new strains of the pathogen for which the immunity was initially developed. After an immunization in a given community, antibodies against an anticipated antigen will be developed and mobilized in the immune system of the immunized population. The mobilized antibodies will specifically bind to the surface proteins of the antigens (pathogens) whenever they appear in the host and deactivate their untoward effects in vivo.

But due to antigenic shift, these antibodies will be ineffective as they will have no binding surfaces. Therefore, the target of most immunization programmes has been to increase the number of individuals immunized against a given disease in a particular population so that these vaccinated persons will help reduce the spread of the infection by protecting those who are not reached by the immunization exercise. This is called herd immunity.

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