In the early years of the AIDS epidemic, epidemiological studies succeeded in establishing the primary transmission routes of a possible AIDS-associated infectious pathogen, later identified as human immunodeficiency virus (HIV), and were able to determine the risk behaviours associated with the disease. With the development of the HIV antibody test a more detailed understanding of transmission was achieved, as the presence of infection could be determined in individuals, and the rate of infection could be determined in categories of people with shared characteristics (risk groups) and, though technically more subject to error, in whole populations. In this way the relative risk of transmission for each behaviour could be measured, refining even further the quantitative assessment of risk of infection.