"By 1950 the nature of the virus was no longer a mystery. Viruses were known to be s, genetic units, parasites that depend on their hosts for and . But a funny thing happened on the road to this knowledge. The viruses that most shaped this emerging portrait were not the most dangerous s, but those examples, however innocuous to humans, that made good laboratory subjects. Researchers constructed general knowledge about viruses based on a few that, by reason of historical precedence or biological robustness, were intensively studied as representatives of the rest."