The prevalence of a disease is the accumulated total of existing cases with respect to the entire population. It is usually reported as the percentage of the population having a particular disease and provides an indicator of how commonly a disease occurs in a population. Disease incidence measures the number of new cases over a certain time period, as compared with the general healthy population. This statistic, also called the case rate or morbidity rate, indicates both the rate and the risk of infection at the time of reporting. The equations used to figure these rates follow:
As an example, let us use a classroom of 50 students exposed to a new strain of influenza. Before exposure, the prevalence and incidence in this population are both zero (0/50). If in a week, 5 out of the 50 people contract the disease, the prevalence is 5/50 = 10%, and the incidence is 1 in 10 (10 is used instead of 100,000 due to smaller population). If within a month 25 more people acquire the flu, the prevalence becomes 30/50 or 60%, and the incidence (the number of new infections) is 25/50, or 5 in 10. Using reported cases of HIV infection as a real-life example, the most recent statistics show a cumulative total of cases (prevalence) between 1981 and 2019 of 1,189,700 cases, or just 0.36% of the population. The incidence of 36,801 cases for just the year 2019 works out to 11.1 cases per 100,000 people.
The changes in incidence and prevalence are usually followed over a seasonal, yearly, and long-term basis and are helpful in predicting trends (figure 1). Statistics of concern to the epidemiologist are the rates of disease with regard to sex, race, or geographic region. Also of importance is the mortality rate, which measures the total number of deaths in a population due to a certain disease. Over the past century, the overall death rate from infectious diseases has dropped, although the morbidity rate, or the number of persons afflicted with infectious dis eases, has remained relatively high.
Fig1. Samples of epidemiological data as analyzed by the CDC. (a) Prevalence of COVID-19, by state, displays total infections per 100,000 persons. (b) Incidence of COVID-19 for a single week displays new cases per 100,000 persons for one week. When viewed on the CDC website, each state can be selected to see a summary of prevalence and incidence data (as seen here for Florida).
Monitoring statistics also makes it possible to de fine how often a disease appears in a given population. An infectious disease that exhibits a relatively steady and predictable frequency over a long time period and is tied to a particular geographic locale is endemic (figure 2a). Often the reason for endemicity is the presence of a reservoir restricted to that location. For example, Lyme disease is endemic to certain areas of the United States where the tick vector is found. New cases are expected to appear in these areas every year. For a disease to be considered sporadic, occasional cases are reported at irregular intervals in widely dispersed locations (figure 2b). Tetanus and typhoid fever are reported sporadically in the United States (fewer than 500 cases a year in a random distribution).
Fig2. Patterns of infectious disease occurrence. (a) In endemic occurrence, cases are concentrated in one region at a relatively stable rate. (b) In sporadic occurrence, a few cases occur randomly over a wide area. (c) An epidemic is an increased number of cases that often appear in geographic clusters. The clusters may be local, as in the case of a restaurant-related food-borne epidemic, or nationwide, as is the case with influenza. (d) A pandemic results when an epidemic ranges across continents and borders.
When statistics indicate that the number of new cases of an endemic or sporadic disease is increasing beyond what is expected for that population, this trend is known as an epidemic (figure 2c). The time period is not defined. It can range from hours in food poisoning to years in STDs, such as gonorrhea or chlamydiosis. The term outbreak is often used interchangeably with epidemic, but an outbreak is usually on a smaller scale in a more limited area. There is no exact percentage of increase required before an outbreak can qualify as an epi demic. Hepatitis C, with more than 3.5 million people in the United States infected and a 150% increase in new cases over the last few years, has certainly reached epi demic proportions.
One useful way to analyze data from outbreaks is to graph the number of cases over time in an epicurve format (figure 3). The pattern of epidemics will ordinarily follow one of three patterns. The first, a point-source epidemic, indicates that the infectious agent came from a single source more or less simultaneously, as when a single batch of contaminated food is served at a barbecue, sickening everyone. The curve be gins abruptly and gradually diminishes. A common source epidemic occurs when all cases came from exposure to the same source, which continues to infect others over time. The epicurve in this case will have irregular peaks corresponding to the timing and extent of exposure. An example is a restaurant worker with poor hygiene who infects diners every time he works. A propagated epidemic occurs when a disease is trans mitted from person to person, as when a microbiology instructor with influenza infects several students during lecture, who in turn infect other students as they attend other classes. The curve in this case shows a sustained increase over time, indicating that it is being communicated from person to person.
Fig3. Epicurve graphs of the three main patterns typically shown by epidemics. McGraw Hill
The spread of an epidemic across continents or international borders is a pandemic, as exemplified by AIDS, influenza, and COVID-19 (figure 2d).
