Plural of virus

In the English language, the plural of virus is viruses.
In reference to a computer virus, the plural is often believed to be virii or, less commonly, viri, but both forms are neologistic folk etymology and no major dictionary recognizes them as alternative forms. These variations, discussed below, are generally used by people in some Internet communities to refer to computer malware of any type, not just self replicating and self distributing forms.
Etymology
No plural in Latin
Virus comes to English from Latin. The Latin word vīrus (the ī indicates a long i) means "poison; venom", denoting the venom of a snake. This Latin word is probably related to the Greek (ios) meaning "venom" or "rust" and the Sanskrit word visham meaning "toxic, poison".
Since vÄ«rus in antiquity denoted something uncountable, it was a mass noun. Mass nouns — such as air, rice, and helpfulness in English — pluralize only under special circumstances, hence the non-existence of plural forms.
It is unclear how a plural might have been formed under Latin grammar if the word had acquired a meaning requiring a plural form.
In Latin vīrus is generally regarded as a neuter of the second declension, but neuter second declension nouns ending in -us (rather than -um) are so rare that there are no recorded plurals. Neuter nouns of other declensions always end in -a (in the nominative, accusative and vocative), but even if we were to apply this rule to vīrus, it would be pure conjecture to guess whether this should give us vīra, vīrua, or something else. There quite simply is no plural for this word in Latin.
Treating vīrus as 2nd declension
If vīrus were a masculine second-declension noun like alumnus, then it would be perfectly correct to use vīrī as its plural. However, it is not, as explained above. In fact, there is a Latin word virī, meaning "men" (the plural of vir, a second-declension masculine noun) but it has a short i in the first syllable: the difference in vowel quantity is reflected in the pronunciation of the English word virile as opposed to viral.
The form vīriī is totally impossible as a plural of vīrus, since we only find -iī in the plural of masculine and feminine words ending in -ius. For instance, if we take radius, we pluralize it by removing the -us and adding -ī. The -iī that we see is not a real ending: it is simply the consequence of adding to a stem that happens to have an i as the last letter. An example of this in English is skiing.
Thus the plural vīriī would be that of the non-existent word vīrius.
Latin nouns in English
Even were the Latin plural known, English-speakers would not be obliged to use it. Examples of Latin loanwords into English which have regular English plurals in -(e)s include campus, bonus, anus and cancer. These stand beside counterexamples such as radius (radii) and alumnus (alumni). Still other words are commonly used with either one: corpus (corpora, or sometimes corpuses), formula (formulae in technical contexts, formulas in more everyday ones).
Use of the form virii
Usage of virii within Internet communities has met with some resistance, most notably by Tom Christiansen, a figure in the Perl community, who researched the issue and wrote what eventually became referred to in various online discussions as the authoritative essay on the subject, favoring viruses instead of virii. The impetus of this discussion was the potential irony that the use of virii could be construed as a claim of superior knowledge of language when in fact more detailed research finds the naive viruses is actually more appropriate.
 
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