HIV-1 at 25.

In 1981, the acquired immunodeficiency syndrome (AIDS) appeared insidiously and mystified doctors and scientists alike. No one could have predicted then that it would become, arguably, the worst plague in human history. Today, 33 million persons are living with infection by the human immunodeficiency virus type 1 (HIV-1), the causative agent of AIDS, while another 25 million have already died of this disease.