Epidemiology is the study of epidemics. Let us now turn our attention to the effects of disease on communities. The term epidemiology really means study of the occurrence of disease; so the epidemiologist is the one who tracks the distribution of disease in a community and gathers information to determine the cause, source, and method of transmission. The results or data gathered can be used to prevent future epidemics and establish necessary treatment programs. Epidemiology is the study of the spread of diseases. What % of diseases can you catch (contract) from someone else? 100%, 75%, 50%, 25%--The answer you might guess is based on your past experience. If you work in an alzheimer’s unit, you may think very few diseases are contagious; however, if you work in a child-care facility, you may think every sick child has an infectious disease and you might "catch" the microbe. Cholera broke out in England in the 1840s. A doctor named James Snow observed the people drinking water from a certain well in London. The well was contaminated by water overflow from an apartment where a cholera patient lived. By eliminating the well as a source of water, the cause of the cholera epidemic was removed; and the epidemic was stopped. For many of us, living through an epidemic is not a reality and seems not to frighten us. However, the AIDS outbreak is evidence that worldwide spread of a disease is still possible. The increase in our freedom to travel throughout the world has increased the possibility that a person can become infected in one country, transmit the microbe to many friends, and not show disease symptoms until later. By law, certain diseases must be reported to health officials. Depending on the state, up to nearly 50 diseases must be reported. Some of these diseases that must be reported include AIDS, botulism, diptheria, gonorrhea, influenza, legionnaire’s disease, lyme disease, malaria, whooping cough, polio, rabies, tetanus, and TB. Individual and collective reporting must protect the privacy of the individual; but cumulative summaries of disease data from the CDC and WHO (Centers for Disease Control and World Health Organization) are used to control epidemics and prevent further outbreaks.