One of the mysteries of the English language finally explained.
Any of a group (now the genus Spumavirus) of retroviruses which infect a number of mammal species (including humans), and which induce the formation of giant cells with vacuolated cytoplasm in tissue culture, but appear to cause little or no disease in their natural hosts.
1950s; earliest use found in Proceedings of the American Philosophical Society. From foamy + virus, so called on account of the vacuolated cytoplasm of infected cells.
foamy virus/ˈfəʊmi ˌvʌɪrəs/
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