Plural of virus
In the English language, the standard plural of virus is viruses. This is the most frequently occurring form of the plural, and refers to both a biological virus and a computer virus.
The less frequent variations viri and virii are virtually unknown in edited prose, and no major dictionary recognizes them as alternative forms. Their occurrence can be variously attributed to hypercorrection formed by analogy to Latin plurals such as alumni or false analogy to Latin plurals such as radii; idiosyncratic use as jargon among a group, such as computer hackers; and deliberate word play, such as on BBSs (see, e.g.: leet).
To complicate matters further, viri is already used in Latin as the plural of vir, meaning "man" (thus making viri mean "men").

# Plural of virus in Latin
The word virus has no classically attested plural form in Latin. In antiquity the word had not yet acquired its current meaning. It denoted something like toxicity; venom; a poisonous, deleterious, or unpleasant agent or principle; or poison in the abstract or general sense.  Since virus in antiquity denoted something noncountable, it was a mass noun.  Mass nouns, such as air, valor, and helpfulness in English, pluralize only under special circumstances, hence the nonexistence of plural forms.
Further, 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 virus is generally regarded to be 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.  Possibilities include vira (by analogy with 2nd declension neuters in -um such as bellum) and virus with a long u (by analogy with 4th declension masculine such as status, although as a neuter noun the plural of virus in the 4th declension would be virua).  However, none of these is attested.
The form virii would not have been a correct plural, since the ending -ii only occurs in the plural of masculine and feminine words ending in -ius. For instance, take radius, plural radii: the root is radi-, with the singular ending -us and the plural -i. Thus the plural virii is that of the nonexistent word virius. The form viri would also be incorrect in Latin. The ending -i is normally used for masculine or feminine nouns, not neuter ones such as virus; moreover, viri (albeit with a short i in the first syllable) is the plural of vir, and means "men."

## Etymology
Virus comes to English from Latin.  The Latin word virus means "poison; venom", denoting the venom of a snake.  This Latin word is probably related to the Greek ios -"to rust" and the Sanskrit word visha -"toxic, poison".

# Use of the form virii
While the word viruses is more often used in medical and professional literature, the form virii 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 occurrences of certain words were bound to attract unwanted attention; the tradition of intentional, sometimes flashy, misspellings originated as a way of communicating semi-steganographically on bulletin boards.
The creation of plural forms by tongue-in-cheek stretching of English plural 'rules' is popular among hackers, sometimes as a way of marking a term as community jargon. See boxen and mouses for the most visible examples. Other examples, whether widely used or not, are easily recognized and deciphered, and it is well understood that these irregular (or hyper-regular) plurals are not errors but examples of geek humor.
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.

# Notes
-  vir stem + i ending from the Latin Dictionary and Grammar Aid, by Kevin Cawley, at the University of Notre Dame, verified 26 February 2005.
-  The first meaning given for this word, a slimy liquid, slime, in the most widely used LatinEnglish dictionaries is inaccurate; the error has been corrected in the more recent Oxford Latin Dictionary.
-  June 1999 issue of ASM News by the American Society for Microbiology
-  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.
-  CHRISTIANSEN, Thomas.  Plural of virus.