A virus that is small enough to pass through a fine-pored filter, as of diatomite or porcelain.
A term with an interesting history. Originally, the term ‘virus’ simply meant a poison or ‘morbid principle’. When it was discovered that an agent capable of causing disease could pass in a liquid through a very fine filter that would hold back bacteria, the agent was called a ‘filterable virus’. In fact, the agent was what we now call a virus-a term derived by the inevitable abbreviation of popular usage.