David Baltimore spent his career working with viruses. His research involved finding the relationship between cancer viruses and the DNA of the cells they infect. In 1975, Baltimore shared the Nobel Prize in Physiology or Medicine with Renato Dulbecco, an Italian-born virologist and Howard M. Temin, an American virologist. In 1965, Temin, an assistant professor of oncology at the University of Wisconsin, proposed for the first time a process called reverse transcription. During this process, viral RNA inserts its own genes into its host cell's DNA. At the Massachusetts Institute of Technology while studying Rauscher mouse-leukemia virus, Baltimore tested Temin's hypothesis and discovered an RNA viral enzyme that alters the host DNA. Temin also found a similar enzyme in the Rous-sarcoma virus. Viruses that alter host DNA in this manner are called retroviruses, and human immunodeficiency virus, the virus that causes AIDS is an example of a retrovirus. The enzyme...