In the English language, the normal plural of "virus" is "viruses". This form of the plural is correct, and used most frequently, both when referring to a biological virus and when referring to a computer virus. The forms "viri" and "virii" are also used as a plural, although less frequently. There is disagreement over whether these forms should be considered correct.

The plural virii is frequently perceived to be founded on a misunderstanding of Latin plurals such as radii. It may have originated as whimsical usage on BBSes (see also: leet). The virii form is used most frequently, although not exclusively, among crackers and computer virus writers with reference to computer viruses. Most computer professionals unaffiliated with the warez, cracker, and virus writing scenes use the "viruses" form instead of the "virii" form.

The viri form is used less often. It is sometimes used by professionals, and can refer to both biological and computer viruses. To complicate matters further, viri is already used in Latin as the plural of vir, meaning "man" (thus making viri meaning "men") [1] (http://www.archives.nd.edu/cgi-bin/lookup.pl?stem=vir&ending=i).

Plural of virus in Latin

The word virus never had a plural form in Latin. In antiquity the word had not yet acquired, of course, its current scientific meaning; rather it denoted something like toxicity, venom, a poisonous, deleterious, or unpleasant agent or principle, or poison in the abstract or general sense¹. Nouns denoting countable entities (e.g. book) pluralize, noncountable entities (e.g. air, mood, valor) pluralize only under special circumstances. The term virus in antiquity appears to have belonged to the latter category, hence the nonexistence of plural forms. [June 1999 issue of ASM News by the American Society for Microbiology]

It is unclear how a plural might have been formed had the word acquired a meaning requiring a plural form ². Possibilities include vira, following the pattern for neuter nouns in –um – or virus with a long "u", following the example of status. However, none of these are attested ³. The virii form would not have been a correct plural, since the "ii" ending only occurs in the plural of words ending in "ius". For instance, take radius, plural radii: the root is radi–, with the singular ending –us and the plural –i. The viri form is also incorrect in Latin. The ending –i is used only for masculine nouns, not neuter ones such as virus; moreover, viri is the plural of vir, and means "men".

Etymology

Virus comes to English from Latin. The Latin word is probably related to the Greek ios and the Sanskrit word visha.

Use of the virii form

While the word viruses is more often used in medical and professional literature, the virii form remains popular in some internet communities. There may be several reasons for the use of this word even when it is known to be unusual.

Leet-speak is the name given to variations on languages where frequent intentional misspellings are common, even using numbers and symbols to replace the letters of a word. These languages developed in an environment where plaintext occurances of certain words were bound to attract unwanted attention, therefore misspellings developed as a way of communicating semi-steganographically on bulletin boards.

The use of plural forms from many different languages is one convenient way to vary the spelling of words without losing comprehensibility, and even today one attribute associated with hackers is a fondness for choosing unusual plural forms. See boxen and mouses for the most visible examples, although any of the 'rules' of English_plurals could conceivably be misapplied to any word without danger of miusunderstanding by hackers.

Debate about the correctness of the "viri" and "virii" forms

The correctness of the viri and virii forms in English is somewhat controversial. The arguments of this debate are closely related to the positions of prescription and description in linguistics.

Some people think that the viri and virii forms are incorrect, since these forms violate the rules of constructing plurals in Latin and are not given as correct plurals by English dictionaries, including specialized dictionaries. This represents a prescriptive view of linguistics. Others, who have a descriptive point of view, think that these forms are acceptable alternatives because they are used with some frequency. From this point of view, the fact that these forms conflict with earlier rules is not relevant.

Notes

1. The first meaning given for this word, a slimy liquid, slime, in the most widely used Latin–English dictionaries is inaccurate; the error has been corrected in the more recent Oxford Latin Dictionary.
2. There is some debate about what the rules of Latin grammar might imply about the formation of a plural. In Latin virus is generally regarded to be a neuter of the second declension, but the word is so rare that there are no recorded plurals. Possibilities include "vira" (in analog with 2nd declension) and "virus" (in analog with 4th declension masculine, although as a neuter noun the plural of virus in the 4th declension would be "virua").
3. To make matters worse, it has been suggested that due to the Latin form of the word, the study of viruses should not be virology (which would be the study of the vir, "man"), but "virulogy". This spelling is extremely uncommon but it is used by a few universities.

References

• The definitive treatise on the plural of “Virus” (mirror) (http://www.linuxmafia.com/~rick/faq/plural-of-virus.html) by Tom Christiansen
• Free On-line Dictionary of Computing entry on virus (http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?viruses)
• The On-line Medical Dictionary entry on virus (http://cancerweb.ncl.ac.uk/cgi-bin/omd?viruses)
• Discussion regarding the plural of virus (http://the-magi.us/stuff/virus.html)
• reference.com FAQ: What is the plural of virus? (http://dictionary.reference.com/help/faq/language/v/virus.html)
• "The plural of 'virus' is 'viruses'" (http://homepages.tesco.net./~J.deBoynePollard/FGA/plural-of-virus.html)