Plural form of words ending in -us
Most English words ending in -us, particularly those derived from Latin, replace the -us suffix with -i to form plurals. This is irregular, however: some words that end in -us do not pluralize with -i. Sometimes this is because they are not Latin words, and sometimes due to habit (e.g. campus, plural campuses, anus, plural anuses, are both Latin words that do not, in English, pluralize with -i). Conversely, some non-Latin words ending in -us or Latin words that would not have pluralized with -i in Latin are given an -i ending in English. Sometimes this plural becomes widely accepted (e.g. cactus, ubiquitously pluralized as cacti), and in other instances would sound unambiguously incorrect to a native speaker (e.g. *ani versus anuses).
In between these two extremes are words that, despite not supporting a Latin plural on etymological grounds, are nonetheless widely pluralized with -i and as such are not immediately heard as incorrect by a substantial number of native speakers (e.g. octopi as a plural for octopus). The question of whether or not these alternative plural forms can be considered incorrect or not touches on the on-going prescriptivism vs descriptivism debate in linguistics and language education.
The -us singular form with an -i plural comes from Latin. However, the morphology of Latin nouns is complex and not every Latin word ending in -us pluralized in -i. The ones that did largely were from the second declension masculine. Briefly, a declension is the way a noun changes to reflect facts about the object to which it refers (e.g., its gender or number) or the relationship that the noun has to other words in the sentence. Remnants of the Old English declension system can be seen in words like I, me, we, and us in modern English, as well as (more distantly) in the 's enclitic. In Latin, just as in many languages spoken today, a word having several forms is the rule rather than the exception. Most words indicate their declension (including their number, i.e. whether they are singular or plural) using an affix, much as we use 's today to indicate possession and (usually) -s to indicate plurality. Specifically, the nominative singular form of second declension masculine nouns is marked with -us, and the nominative plural with -i. More at Latin grammar. Confusion arises because some Latin words ending in -us would not have pluralized with -i. Examples of regularly declined nouns include third declension neuters, such as opus, with the plural opera, and fourth declension masculine and feminine, such as sinus and tribus, with plurals sinūs and tribūs. Some idiosyncratic instances are bus, a curtailed form of omnibus, a dative plural, and ignoramus, a verb.
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 Internet, 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 turns out not to be straightforward, as no plural form is attested in extant Latin literature. Furthermore, its unusual status as a neuter noun ending in -us apparently not of Greek origin obscures its morphology, making guesses about how it should have been declined difficult.
 Mass noun in 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 ἰός (ios) meaning "venom" or "rust" and the Sanskrit word viṣam meaning "toxic, poison".
Since vīrus 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 in the texts.
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. Plural 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 conjecture whether this should give us vīra, vīrua, or something else. There simply is no known plural for this word in Classical Latin.
In Neo-Latin, a plural form is necessary in order to express the modern concept of ‘viruses’. Dictionaries such as Whitaker's Words therefore treat it as a second-declension noun with the following fairly ordinary forms:
 Treating vīrus as 2nd declension masculine
If vīrus were a masculine second declension term like alumnus, it would be correct to use vīrī as its plural. However, it is not a masculine second declension term, as was explained.
There does exist 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 different pronunciations of the (Latin-derived) English words virile and viral.
The form vīriī is impossible as a plural of vīrus since we only find the ending -iī in the plural form of masculine and feminine words ending in -ius. For instance, radius is pluralized by removing -us, to isolate the stem radi, and then adding the plural suffix -ī. Thus the -iī ending of the resulting word radiī is not a suffix: it is simply the consequence of adding the actual suffix ī to a stem that has an i as its last letter. An example of this in English is adding the suffix ing to the word ski, resulting in skiing. Vīriī would be the plural form of the putative, nonexistent word vīrius.
 Latin nouns in English
Even if the Latin plural were 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).
As an indication of usage, viruses appears in the official Scrabble words list, but neither viri nor virii does. Similarly, the spellchecker built into the Mozilla Firefox browser accepts viruses but neither viri nor virii.
 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. (See hypercorrection.)
In life sciences, "viruses" generally refers to several distinct strains or species of virus. "Virus" is used in the original way as an uncountable mass noun, e.g. "a vial of virus". Individual, physical particles are called "virions" or simply "virus particles".
There are three plural forms of octopus: octopuses [ˈɒktəpəsɪz], octopi [ˈɒktəpaɪ], and octopodes [ˌɒkˈtəʊpədiːz]. Currently, octopuses is the most common form in the UK as well as the US; octopodes is rare, and octopi is often objectionable.
The Oxford English Dictionary lists octopuses, octopi and octopodes (in that order); it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun, which it is not. Rather, it is (Latinized) Ancient Greek, from oktṓpous (ὀκτώπους), gender masculine, whose plural is oktṓpodes (ὀκτώποδες). If the word were native to Latin, it would be octōpēs ('eight-foot') and the plural octōpedes, analogous to centipedes and mīllipedes, as the plural form of pēs ('foot') is pedes. In modern Greek, it is called khtapódi (χταπόδι), gender neuter, with plural form khtapódia (χταπόδια).
Chambers 21st Century Dictionary and the Compact Oxford Dictionary list only octopuses, although the latter notes that octopodes is "still occasionally used"; the British National Corpus has 29 instances of octopuses, 11 of octopi and 4 of octopodes. Merriam-Webster 11th Collegiate Dictionary lists octopuses and octopi, in that order; Webster's New World College Dictionary lists octopuses, octopi and octopodes (in that order).
The term octopod (plural octopods or octopodes) is taken from the taxonomic order Octopoda but has no classical equivalent. The collective form octopus is usually reserved for animals consumed for food.
The situation with the word platypus is similar to that of octopus; the word is etymologically Greek despite its Latinized ending, and so pluralizing it as if it were Latin (i.e. as platypi) is ill-considered. As with octopus, importing Greek morphology into English would have platypodes as the plural, but in practice this form is not well-attested. In scientific contexts biologists often use platypus as both the singular and plural form of the word, in the tradition of sheep or fish, but laypersons and scientists alike often use the simple English plural platypuses. There is no consensus on which of these two is correct, with different dictionaries making different recommendations.
 Botanical Latin
As a word in Botanical Latin (as distinct from Classical Latin), cactus follows standard Latin rules for pluralization and becomes cacti, which has become the prevalent usage in English. Regardless, cactus is popularly used as both singular and plural, and is cited as both singular and plural. Cactuses is also an acceptable plural in English.
 Facetious formations
Facetious mock-erudite plurals in -i or even -ii are sometimes found for words ending with a sound (vaguely) similar to -us. Examples are stewardi (supposed plural of stewardess) and Elvii (as a plural for Elvis imitators). The Toyota corporation has determined that their Prius model should have the plural form Prii, even though the Latin word prius has a plural priora, the Lada Priora having prior claim to that name — though the common plural is "Priuses". The Winklevoss twins are sometimes collectively referred to as "the Winklevii".
- A Dictionary of Modern English Usage (1950) H. W. Fowler, Oxford University Press
- VLAD Magazine, Issues #1, #3, #5, #7
- Chambers's Etymological Dictionary Enlarged Edition 1931
- June 1999 issue of ASM News by the American Society for Microbiology
- OSW Official Scrabble Words (1989) Chambers
- Mozilla Firefox web browser v3.5.7, 2009, with the standard US-English dictionary
- Tom Christiansen (17 December 1999). "What's the Plural of 'Virus'?". Rick Moen. Retrieved 29 December 2010.
- Peters, Pam (2004). The Cambridge Guide to English Usage. Cambridge: Cambridge University Press. ISBN 0-521-62181-X, p. 388.
- Oxford English Dictionary 2004 update (subscription required). Retrieved October 22, 2007.
- Chambers 21st Century Dictionary. Retrieved October 19, 2007.
- Compact Oxford Dictionary Retrieved October 19, 2007.
- Random House Unabridged Dictionary (2006)
- engadget, "Toyota decrees the plural of 'Prius' is 'Prii,' your Latin teacher looks on admonishingly", Tim Stevens , 21 February 2011
- "GREEN: Toyota Readies Fleet Of Priuses", John Voelcker
- Vanity Fair, "The Code of the Winklevii", Dana Vachon , December 2011
- "What is the plural of virus?". reference.com FAQ. Retrieved 2 January 2005.