Patient Zero -- 
Patient Zero is the first known person with AIDS. Patient Zero (aka The Index Patient) is not necessarily or even usually the first person to be diagnosed. Often Patient Zero is dead before health authorities get to the scene. Instead, Patient Zero is the person from whom all subsequent infections are received, directly or more likely indirectly. Think of it as a telephone tree, with Patient Zero at the head. In the case of AIDS, it's believed it was a Canadian Airlines worker who caught the disease in Africa and brought it back to Canada, where it quickly spread to the U.S. Note that in this case his Patient Zero status really only applies to North America, since he was probably not responsible for the hundreds of thousands (millions?) of cases in Africa and other countries.