It is thought that viruses and their hosts must constantly adapt to each other to survive. A striking example of this phenomenon is revealed in a comparison of the genetic evolution of primates with that of simian foamy viruses. These ubiquitous nonpathogenic retroviruses infect all primates with the curious exception of Homo sapiens. The study shows that these viruses have co-speciated with Old World primates for at least 30 million years, making them the oldest known vertebrate RNA viruses. Humans are susceptible to simian foamy viruses and have a common evolution with Old Word monkeys, so the apparent lack of a human foamy virus provides interesting avenues for speculation.