Epidemiology is a fundamental population science and tries to answer questions about health-related behaviors and outcomes in populations. Epidemiologists study how health and disease are distributed in populations and the factors that influence or determine those distributions. Why does a disease develop in some people and not others? Why are some people healthier than others? The premise underlying epidemiology is that disease and health-related factors are not randomly distributed in a population. Rather, each of us has certain characteristics, life situations and habits that predispose us, or do not predispose us, to states of health and illness.