Epidemiology is defined as the study of:
where and when a disease occurs, and how it is transmitted.
when a disease occurs.
how a disease is transmitted.
where a disease occurs.

Where and when a disease occurs and how it is transmitted

Explanation:

Epidemiology is the branch of health science related with the study of all the factors responsible for the presence and spread of a disease. It answers the questions like; where the disease is present and most common, mode of transmission, signs and symptoms, comparisons of treatments etc. It attempts to use the information gotten from this study to the control of the disease and other health problems