Epidemiologists are scientists that study the factors influencing the health status of populations. These populations may be defined by geography (such as the residents of a particular city), occupation (such as members of the armed forces), or any other common trait (such as age, race, or sex). Epidemiologists look for trends in measures of the health of the population, such as the average life span, the leading causes of death, and the number of cases of a disease that are found in the population. To determine what causes certain trends or health problems, epidemiologists collect large amounts of data about individuals in the population. They analyze these data to determine who is sick, when they got sick, and what factors the sick people have in common. The process is similar to that which investigators use to search for clues to solve a crime. Thus, epidemiologists are often described...