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This question already has an answer here:

What is the plural of "virus"? I have seen the word "viri" floating around in various sources throughout my life, but most people say "viruses". The spelling checkers I have tried do not recognize "viri", but I have learned that is not a very good method of verifying a word.

marked as duplicate by curiousdannii, ab2, k1eran, Ste, Edwin Ashworth Oct 17 '16 at 9:56

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The Wikipedia article on Plural form of words ending in -us has a long section on viruses vs viri.

Below, are quoted the first few paragraphs:

The English plural of virus is viruses. In most speaking communities, this is non-controversial and speakers would not attempt to use the non-standard plural in -i. However, in computer enthusiast circles in the late 20th century and early 21st, the non-standard viri form (sometimes even virii) was well-attested, generally in the context of computer viruses.

While the number of users employing these non-standard plural forms of virus was always a proportionally small percentage of the English-speaking population, the variation was notable because it coincided with the growth of the web, a medium on which users of viri were over-represented. As the distribution of Internet users shifted to be more representative of the population as a whole during the 2000s, the non-standard forms saw decline in usage. A tendency towards prescriptivism in the computer enthusiast community, combined with the growing awareness that viri and virii are not etymologically supported plural forms, also played a part.

Nonetheless, the question of what the Latin plural of virus would have been in ancient times turns out not to be straightforward, as no plural form is attested in ancient Latin literature. Furthermore, its unusual status as a neuter noun ending in -us and not of Greek origin obscures its morphology, making guesses about how it should have been declined difficult.[...]

The article continues with discussion on Latin plurals.

  • A quick search for "virus virii" turns up more than one duplicate. – Edwin Ashworth Oct 17 '16 at 9:59
  • @edwin-ashworth I hadn't realised when I answered (from my phone). Anyway I have also marked it as duplicate - I don't see any benefit in deleting this answer so will leave it in case useful to someone. – k1eran Oct 17 '16 at 10:10
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According to the Wiktionary,virus has been

Borrow[ed] from Latin virus ‎(“poison, slime, venom”).

From the same page, virus is at the same time countable and uncountable. Its plural forms are viruses or vira, and viri and virii are proscribed.

Note: from that page, viri exists in Latin but is the nominative plural of vir which means man.

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