Harold Jaffe's new article "The early days of the HIV/AIDS epidemic in the USA" emphasises the power of the epidemiological method to gain an understanding of disease pathogenesis by recounting some of the work done before the isolation of HIV. He says,

Although the importance of [the discovery of HIV] cannot be underestimated, it is also important to understand the epidemiological work that preceded the discovery. It was this work that established AIDS as being most likely caused by a transmissible agent, defined the transmission routes of the agent, suggested its natural history and also provided the basis for initial prevention guidelines.

Jaffe, W. The early days of the HIV-AIDS epidemic in the USA. Nature Immunology 9, 1201 - 1203 (2008). (If you don't have access to a Nature subscription, the full article can be read here.)