so called because most viruses are capable of passing through fine filters that bacteriacannot pass through
filterable virus in American English
noun
a virus particle small enough to pass through a filter of diatomaceous earth or porcelain, which will not pass bacteria: chiefly historical or an informal indicator of size, as synthetic membrane filters now permit passage of the smallest virus
Word origin
[1910–15]This word is first recorded in the period 1910–15. Other words that entered Englishat around the same time include: carbon cycle, flashback, payoff, radio, spotlight