The term “epidemiology” is derived from the Greek word that means frequency. The frequency or prevalence of a disease is the total number of existing cases in a given population. Incidence is the number of new cases during a particular time; incidence rate is also referred to as morbidity.

As an example let’s use microbiology during winter quarter which has 100 students. Prior to the flu season, the prevalence and incidence were both zero (0/100). During the first week of class 5 out of the 100 students contract the flu, so the prevalence is 5/100 = 5% and the incidence is 1:19 (5:95). If during the second week 15 more students get the flu; what is the prevalence and incidence? (prevalence = 20%; incidence = 3:16) Is that what you thought? In considering larger populations, the incidence is usually given in number of cases per 1000 or 100,000. Neither prevalence or incidence indicate the number of deaths which is called the mortality rate. During the last 100 years, the mortality rate for most infectious diseases has dropped, even though the incidence rates may remain high. Why?

Public health officials must keep good records of the prevalence and incidence of cases of persons with infectious diseases. Keeping the incidence of a disease low reduces human suffering and financial loss due to loss of work. Incidence of disease is also important to animal scientists, agronomists, and conservation (natural resources) officers.

The soil and native plant and animal populations will always serve as a reservoir for potentially pathogenic microbes, so some diseases will probably never be eliminated. Some human diseases like gonorrhea and measles rely on the human body as their microbe reservoir so it is possible to eliminate these diseases. Can you name a disease thought to have been eliminated? Smallpox virus is thought to have been eradicated from the earth.

An infectious disease that exhibits a relative constant low incidence rate that is acceptable to health officials is called endemic. In northeastern Minnesota where the tick vector is found, there will be an expected number of new cases of lyme disease every year. When an endemic disease appears to be increasing beyond what is expected and indicates potential danger, harm, or increased mortality, the disease is called epidemic. Several factors are considered before a disease is considered an epidemic----type of microbe, density of human population, location of outbreak, level of known vaccinations, and time of year. For instance six cases of spinal meningitis may be an epidemic in Mankato, Minnesota; but 60 new cases may be needed before health officials in east Asia become concerned.

Pandemic is used to describe epidemics that spread to new continents (worldwide) or break out in different parts of the world at the same time. Ancients believed that the worldwide pandemics of influenza (pneumonial flu) were related to the celestial alignment of the stars. Five pandemics of influenza have occurred in 1900, 1918, 1946, 1957, and 1968. At least 50 million people died in the 1918 pandemic outbreak. Even today influenza ranks in the top 10 causes of death in the world. Did you have your flu shot? What is the other pandemic disease of the 1990s? (AIDS) One of the first pandemics recorded was the plague in the 1300s. In some diseases like Salmonella food poisoning, it is estimated that only 1-10% of the actual cases are reported. Reporting of sexually transmitted diseases and colds probably is also very incomplete so the incidence of these diseases may be much higher than statistics reveal.