Disease refers to an abnormal condition in any living organism that causes disorder in structure and disrupts the normal functioning of all or part of the body, often accompanied by distinctive signs and symptoms.
In the fields of epidemiology and public health, the adverse effects of such health outcomes are observed and quantified using various tools known as measures of disease frequency.
These measures help determine how frequently a disease or health event occurs.
Understanding the frequency of disease and how it changes over time and across different population subgroups is vital for identifying disease causes and developing effective prevention strategies.
In epidemiological research, there are three levels of response: describing the spread of the health outcome with respect to the population's characteristics and its occurrence over time; identifying and establishing the determinants (factors) responsible for the disease; and evaluating the impact of proposed measures for treatment.
Selecting the appropriate research measures for each level of response is critical and depends on both the specific research question and the data available.
Counts
Counts represent the fundamental measure of disease frequency, involving the simple tallying of the number of people who have a particular disease within a specific period.
For example, the total number of cholera cases in Biratnagar city was 160 in 2021 and 170 in 2022.
While counts are essential for identifying trends, conducting surveillance, and tracking the occurrence of health events, the information they provide is limited.
This is because counts do not take into account the size of the population at risk or allow for comparison between different populations or time periods.
Ratio, Proportion and Rates
A ratio is a comparison between two independent quantities expressed as a fraction, where the numerator and denominator are mutually exclusive, meaning one quantity is not part of the other.
Ratios are used both as descriptive measures and analytical tools in epidemiology.
For example, the ratio of the male to female population affected by diabetes is 2/1, expressed as 2:1.
A proportion is a specific type of ratio where the numerator is a part of the denominator, making it a descriptive measure that compares a part to the whole.
Proportions can be expressed as a decimal, fraction, or percentage.
For instance, if the number of diabetic men is 150 and the total number of men is 560, the proportion of diabetic men is (150/560) × 100 = 26.78%.
A rate is defined as the frequency at which an event occurs over a specified period in a population at risk.
It serves as a measure of risk and is especially valuable for comparing disease frequency across different locations, time periods, or population groups that vary in size and characteristics.
Incidence and Prevalence
Morbidity refers to any deviation—either subjective or objective—from the state of physical and psychological well-being, including diseases, injuries, and disabilities.
In epidemiology, morbidity considers the number of individuals affected, the duration of illness within the population, or the periods of illness experienced.
Incidence and prevalence are the most commonly used measures to quantify the frequency of morbidity.
Incidence
These two terms are often confused, so understanding their key differences is essential.
Incidence refers to the rate at which new cases of a disease occur in a specified population during a given time period.
It is calculated by dividing the number of new disease cases in a specific period (e.g., one year) by the number of individuals initially free from the disease in the population.
For example, the annual incidence of meningococcal disease in Auckland, New Zealand is 16.9 per 100,000 people.
Incidence Proportion, also called Cumulative Incidence or Risk, represents the probability that an initially disease-free individual will develop the disease within a specified time period.
Incidence Rate, also known as Person-time rate or Incidence Density, measures the number of new cases relative to the total time each person was at risk during the study period.
Prevalence
Prevalence indicates the proportion of existing cases of a disease within a specific population at a particular point or over a period of time.
It is calculated by dividing the current number of disease cases by the total population examined at that time.
For example, if 735 out of 3000 individuals are currently affected by HIV/AIDS, the prevalence is 735/3000 = 0.245 or 24.5% when expressed as a percentage.
Point Prevalence measures the proportion of individuals affected by a disease at a specific point in time.
Period Prevalence measures the proportion of individuals affected by a disease over a specified period.
Difference between Incidence and Prevalence
Incidence is the rate or proportion of new cases within a population during a specific time period, whereas Prevalence is the proportion of existing cases within a population at a given point or over a period of time.
Incidence is a dynamic measure of disease frequency, while prevalence is a static measure.
Incidence includes only new cases, whereas prevalence includes both new and pre-existing cases.
Incidence is primarily used to assess risk factors and to investigate causes and determinants of disease occurrence.
Prevalence is mainly used to describe the disease burden, especially for chronic diseases with long durations and varying onset times.
Cumulative Incidence ranges from 0 to 1, and Incidence Rate ranges from 0 to infinity.
Cumulative Incidence is interpreted as probability, and Incidence Rate is interpreted as the reciprocal of time at risk.
Prevalence is interpreted as a proportion within the population.
Mortality Rates
Mortality refers to the frequency of deaths resulting from a disease.
Life expectancy at birth is the average number of years a newborn is expected to live, assuming that current age-specific mortality rates remain constant throughout their life.
Life expectancy serves as a key indicator of general mortality in a population and reflects the effectiveness of public health infrastructure and health care systems.
The mortality rate is defined as the total number of deaths from a particular disease within a defined population over a specific time interval.
For example, if 200 deaths due to typhoid occur in a population of 500 people, the mortality rate is 200/500 = 0.4 or 40%.
Various measures of mortality based on population characteristics and demographics include:
Crude Mortality Rate / Crude Death Rate
Case-Specific Death Rate
Neonatal Mortality Rate
Post-neonatal Mortality Rate
Infant Mortality Rate
Maternal Mortality Rate
Proportionate Mortality
Death-to-case ratio
Case Fatality Rates
Case Fatality Rate (CFR) is the ratio of deaths due to a specific disease to the total number of diagnosed cases of that disease within a given time period.
For example, in a population of 1,000 people, if 500 are diagnosed with a specific disease and 150 die from it, the case fatality rate is 150/500 = 0.3 or 30%.
CFR provides an estimate of the risk of dying from a disease and is commonly used to assess the effectiveness of treatments.
As treatments improve, case fatality rates are expected to decrease.
CFR can vary across populations and time periods due to factors such as the causative agent of the disease, host characteristics, environmental conditions, and the availability or quality of treatment.
Attack Rates
Attack Rate (or attack risk) is a measure used in epidemiological studies, particularly during outbreak investigations with relatively short observation periods.
It represents the proportion of individuals who develop a disease, based on case definition (such as clinical symptoms or serological tests), among a population at risk at the start of the defined time interval.
Attack rate is particularly useful for identifying exposures that contribute to acute disease outbreaks, such as the consumption of a specific food item
It helps in quickly assessing the extent of disease spread and identifying potential sources.
Various types of attack rates include:
Overall Attack Rate – proportion of all individuals at risk who become ill during the outbreak.
Food-Specific Attack Rate – proportion of individuals who consumed a specific food and became ill, used to identify foodborne illness sources.
Secondary Attack Rate – proportion of susceptible individuals who become infected after exposure to a primary case, commonly used to measure the spread within households or close-contact groups.
References
Centers for Disease Control and Prevention. (n.d.). Lesson 3: Measures of Risk — Section 1. CDC. Retrieved from https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section1.html
Centers for Disease Control and Prevention. (n.d.). Lesson 3: Measures of Risk — Section 2. CDC. Retrieved from https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
Centers for Disease Control and Prevention. (n.d.). Lesson 3: Measures of Risk — Section 3. CDC. Retrieved from https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section3.html
Noordzij, M., Dekker, F. W., Zoccali, C., & Jager, K. J. (2010). Measures of disease frequency: Prevalence and incidence. Nephron Clinical Practice, 115(1), c17–c20. https://doi.org/10.1159/000286345
Jones, R., & Finlay, I. (2000). Prevalence and incidence: What is the difference? Students 4 Best Evidence (S4BE). Retrieved from https://s4be.cochrane.org/blog/2020/11/06/prevalence-vs-incidence-what-is-the-difference/
The BMJ. (n.d.). Chapter 2: Quantifying disease in populations. In Epidemiology for the uninitiated. Retrieved from https://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated/2-quantifying-disease-populations
Encyclopedia Britannica. (n.d.). Mortality (demography). Retrieved from https://www.britannica.com/science/mortality-demography
Encyclopedia Britannica. (n.d.). Attack rate. Retrieved from https://www.britannica.com/science/attack-rate
Blackwell Publishing. (n.d.). Measures of disease frequency. Retrieved from https://www.blackwellpublishing.com/specialarticles/jcn_9_188.pdf
