There are two types of epidemiology: descriptive and analytic. Descriptive epidemiology aims to describe the distribution of people who gets sick (versus those who don't) with regards to time, place, and person (TPP). TPP can also be thought of as answering the questions of when, who, and what respectively; in other words it investigates when a health issue or disease first emerged in a population, who in the population are getting sick, and where the disease/health condition seems to have first originated and spread from. Analytic epidemiology then uses the data provided by TPP to make and test hypothesis which aim to determine the cause of an outbreak or disease cluster among a population. Epidemiologist evaluate morbidity (people who are sick or injured) and mortality (people who are killed) to understand the burden of disease among populations.