Then suddenly there were 5 cases in one community in one year. PCP was the disease that alerted the Center for Disease Control that something strange was happening in the gay community, and the first signs of the new mystery illness. Today, PCP is common in AIDS patients, and still very rare in the general uninfected population. Mycobacterium Avium Complex, or MAC for short, is another AIDS-defining disease. This one is caused by bacteria related to tuberculosis.
Scientists think it is transmitted in soil or water, but no one knows for sure.
However, the bacterium doesn't normally cause illness, except in the absence of an immune system. Before 1984, the US medical records knew of only 37 cases. Now, most AIDS patients with CD4 counts of less than 50 will get the disease. Of course, AIDs patients get diseases which affect the general population too. They just get them worse and often have different symptoms.