In 1981, an obscure and deadly disease surfaced. Previously healthy homosexual men in the United States began arriving at clinics with rare cancers and infections usually seen in people with weakened immune systems. Most of them died. The medical community was baffled and the public anxious. As the cases multiplied, so did the questions. Who is at risk? What is causing the disease? Why does it lead to failure of the immune system? And most important: Can it be stopped from spreading? The new disease was named acquired immune deficiency syndrome, or AIDS, and it has now killed more than 25 million people worldwide. After the initial outbreak, different sectors of the public health, medical, scientific, and advocacy communities mobilized in response to the deadly epidemic. They focused on surveillance—that is, detecting and mapping the disease—and prevention.