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A virus that is small enough to pass through a fine-pored filter, as of diatomite or porcelain.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
a virus small enough to pass through a bacteria-retaining filter: an informal indicator of size, as recent filters can hold back the smallest viruses.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.