Chapter 10 Simian Foamy Viruses: Infections in Human and Nonhuman Primate Hosts Carolyn R. Stenbak, Delia M. Pinto-Santini, Shannon M. Murray, and Maxine L. Linial Abstract Foamy viruses are ancient and ubiquitous retroviruses that infect a variety of mammalian hosts. In this chapter, we focus on foamy viruses that infect nonhuman primates (NHP), called simian foamy viruses or SFV. Natural SFV infection in monkeys and apes leads to life-long, persistent infections with no associated patho- genicity. Although SFV have coevolved with their natural hosts and show strong cospeciation, there are also many examples of cross-species transmission events. SFV are transmitted primarily via saliva, and humans who come into contact with NHP saliva can become zoonotically infected with SFV. To date, SFV from a variety of NHP species have been transmitted to humans and, as seen in natural infections, there is no pathogenicity associated with these zoonotic infections. However, as in the case of other retroviruses, such as lentiviruses, it is possible that an SFV viral variant could emerge as a human pathogen. The molecular features of SFV, the situations that lead to SFV zoonotic infections, and the implications of these infections are discussed in the global context of the monkey–human interface. Keywords Zoonotic transmission · Retrovirus · Foamy virus · Recombination · Gene therapy vectors 10.1 Introduction to Virology and Retrovirology Viruses are the most abundant biological entities on Earth (Edwards and Rohwer 2005). They have been found to infect all known life forms, including bacteria, fungi, plants, and animals. Viruses are entirely dependent on host cells for their C. R. Stenbak Department of Biology, Seattle University, Seattle, WA, USA D. M. Pinto-Santini · S. M. Murray · M. L. Linial (*) Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA, USA e-mail: mlinial@fredhutch.org © Springer Nature Switzerland AG 2020 S. Knauf, L. Jones-Engel (eds.), Neglected Diseases in Monkeys, https://doi.org/10.1007/978-3-030-52283-4_10 233