Hilleman collected blood from gay men and intravenous drug users—groups known to be at risk for viral hepatitis. This was in the late 1970s, when HIV was yet unknown to medicine. In addition to the sought-after hepatitis B surface proteins, the blood samples likely contained HIV. Hilleman devised a multistep process to purify this blood so that only the hepatitis B surface proteins remained. Every known virus was killed by this process, and Hilleman was confident that the vaccine was safe.
