The phrase "foamy virus" refers to a type of virus that is named after the way it looks under a microscope. It has a foamy appearance because of the bubbles or foam-like structures seen on its surface. Full definition
Novel functions of prototype foamy virus Gag glycine - arginine - rich boxes in reverse transcription and particle morphogenesis. (dresden-ipp.de)
Yu - Ping Ho (Lindemann, TUD)-- «Characterization and Manipulation of Foamy Virus Capsid - Cell Interactions» (2013) (dresden-ipp.de)
The simian foamy virus — so - called because of its foamy appearance under a microscope — is one of three known retroviruses in nonhuman primates. (sciencemag.org)