Terrestrial Epidemiology [and Discussion]

Although there are many lacunae in our understanding of the origins and mode of spread of infective disease, it does not appear that the lacunae can be most economically filled by invoking a recurrent extraterrestrial origin for the microbes. For many infections, there is no need, since the pathogens are always demonstrable in the community, and the striking variations in epidemic capability are probably explicable by changes - including lysogenization with phage - in the existent population rather than by recruitment from without. Influenza undoubtedly has many epidemiological perplexities but the very subtle secular variations in antigenic structure, referred to as antigenic drift, seem most rationally explained by evolution in human hosts on Earth.