Epidemiology study is the study of diseases, conducted among populations of humans. Such a study highlights HOW the diseases happen, WHEN the diseases occur and WHERE the diseases occur.
The information gathered is necessary to strategize on how to stop the spread of the diseases, to prevent ailment and also as a guide to the care of the patients.
Epidemiologists also attempt to determine what factors are associated with diseases (risk factors), and what factors may protect people or animals against disease (protective factors).
Let's say you enjoy swimming. One day you decide to swim alone. But it's a risky idea because you could drown and no one would be there to help you in time. So it's best to have others around incase you have an accident or need help.