What is Epidemiology? - Definition & Medical Terms

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  • 0:01 Epidemiology
  • 0:58 Epidemic
  • 3:04 Endemic
  • 5:51 Pandemic
  • 7:55 Lesson Summary
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Lesson Transcript
Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

We are going to gain an understanding of epidemiology and what an epidemiologist is looking for. We will also explore some common terms and examples of past epidemiological studies.


Have you given much thought about the different diseases that exist? Or have you wondered which country or state has the most diseases?

You've probably at least thought a little about the diseases that you think could possibly affect you and your family. But did you know there is a field of science that keeps up with the various diseases and overall health status of people? Epidemiology is the study of patterns, occurrences, and control methods of diseases and other factors that determine health status. The scientist who studies epidemiology is known as an epidemiologist.

Epidemiologists study all diseases to look for any trends. As they are studying, they may notice particular patterns with the spread of diseases or factors affecting health. They may even notice that some diseases don't have patterns at all. Let's look at the three main patterns that epidemiologists may identify.


It was very difficult to go one day during the year 2014 without hearing something about Ebola in the news or on social media. It was one of the most frequently searched and discussed topics of the year. Ebola is a deadly infectious disease, and it was spreading quickly.

Many people in the U.S. were very afraid of Ebola, especially once people infected with the disease were brought from other countries into the U.S. for treatment. When a disease occurs suddenly and affects a disproportionately large number of people in a given area or a population at one time, it is referred to as an epidemic.

The Ebola epidemic, also known as Ebola hemorrhagic fever or Ebola virus disease, can be caused by any of a few viruses belonging to the Filoviridae family of viruses. Ebola was first discovered in 1976 in Africa.

Since then, there have been outbreaks of Ebola, but the outbreak in 2014 was the biggest and most widespread. The 2014 epidemic started in West Africa and spread from there. This epidemic has claimed thousands of lives, with a fatality rate ranging from 55-90% of those infected.

Let's look at another epidemic from a long time ago. One of the most severe epidemics ever in U.S. history was the yellow fever epidemic of 1793. This epidemic occurred in Philadelphia, PA, thanks to mosquitoes that carried the yellow fever virus. The virus was spread through mosquito bites and quickly affected thousands of people.

The population in Philadelphia at that time was about 45,000. Out of that, approximately 5,000 died from the yellow fever epidemic. It is believed that the only reason more people didn't die from the disease is because about 20,000 people left the city to avoid it. This epidemic started around July of 1793 and ended in November of that year once the mosquitoes died out due to winter conditions.


So, you know what an epidemic is; now let's discuss the next pattern. A disease may also be an endemic. This is when a disease is regularly found within a certain population of people or in a certain area, and it is predictable. There are two endemics that you are probably familiar with, but just didn't know that they were considered to be endemics.

Chicken pox is an endemic that is all too familiar to people who grow up in the U.S. and even some other countries. Chicken pox is an endemic because it mostly affects school-aged children. It is rare to hear of adults having this disease. It is also mostly seen in the Northern Hemisphere. Epidemiologists know that chicken pox breakouts occur during winter and early spring because school is in session and children are indoors more often due to the colder weather.

Chicken pox was first discovered in the 1500s. It's caused by the varicella zoster virus and is spread through droplets in the air after an infected child sneezes or coughs. Almost every child in the U.S. used to get chicken pox prior to the vaccine being approved. It only resulted in about 100 deaths per year, so it is usually not deadly.

However, epidemiological studies have shown that the disease has a higher incidence of causing death the older a person is when they catch it. As a matter of fact, about 50 of the 100 deaths from chicken pox each year are from adults, even though adults make up only about 5% of the cases of chicken pox each year. The vaccine schedule for this disease was set based on the results of epidemiological studies, which has greatly reduced the number of chicken pox cases each year.

Another well-known endemic is malaria. This disease is seen regularly in most countries of Africa and other tropical regions close to the equator. Epidemiologists are able to fairly accurately predict the numbers of malaria cases that will occur each year. The vast majority of cases are usually seen in sub-Saharan Africa.

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