Rotello then describes how HIV entered this system and began to spread. He cites data from a large study of Hepatitis B transmission among gay men that was enrolled in 1978, testing thousands of gay men in several American cities at 6-month intervals, a study which included collecting and freezing their blood samples for later study. The samples, “...were to become invaluable measures of the rise of HIV infection once a test for HIV antibodies became available in the mid-eighties. Researchers were then able to go back, retest the blood, and obtain a stop-motion picture of HIV’s deadly incursion into the gay male population.” The results indicate that when the study began in 1978 the percentage of gay men infected with HIV was in the low single digits. By 1985 it had risen to between 40% and 60% in most test cities, with peaks of 58% in Denver and Seattle, 60% in San Diego, 70% in Philadelphia, and 73% in the San Francisco cohort.