I've gussied up Dr. Snow's map for ease of reading in a web context. Click on the image to see the real map at WikiCommons.
British physician John Snow invented the science of epidemiology with this map. It shows the location of deaths from a London cholera outbreak in 1854. The map was sufficient proof for city officials to remove the Broad Street pump's handle, ending the outbreak. His subsequent statistical analysis of this and other outbreaks proved that a water borne agent was responsible for the disease. This fitted nicely with the new germ theory of infection.
Its only 150 years since we began to understand the actual causes of disease. Disease has been the single biggest cause of human misery. Smallpox killed at least 300 million in the 20th century. It is only a recent phenomenon that parents could expect all their children to reach adulthood. And that's by no means true everywhere. But the situation improves slowly, steadily in many parts of the world. Every year that fewer parents have to bury their children as a matter of routine is a better year than the one before.
Anyway, epidemiology is the profession closest to detectives as portrayed in movies and books. Anyone interested in the type of work shown on CSI ain't going to find it at their local police department. Better to go into epidemeology where the real detectives work.
CDC page on Cholera
CDC Disease Index
Wikipedia page on Infectious Desease