"When reports emerged 30 years ago that young gay men were suffering from rare forms of pneumonia and cancer, public health investigators scrambled to understand what appeared to be a deadly immune disorder: What were the symptoms? Who was most susceptible? What kinds of infections were markers of the disease?
They were seeking the epidemiologist's most essential tool an accurate case definition, a set of criteria that simultaneously include people with the illness and exclude those without it. With AIDS, investigators soon recognized that injection-drug users, hemophiliacs and other demographic groups were also at risk, and the case definition evolved over time to incorporate lab evidence of immune dysfunction and other refinements based on scientific advances. "