Plural of “virus”

The plural of virus is viruses.

Why

"Virus" is Latin for "poison" or "slimy liquid," a word that in classical Latin functioned essentially as an uncountable mass noun, since Romans didn't typically count individual instances of poison the way modern science counts individual viral strains. Because classical Latin never developed a true plural for the word in ordinary use, English had no inherited Latin plural to draw on when it adopted "virus" for the biological sense in the late 19th century, and simply built a regular English plural instead.

This puts "virus" in a similar position to "referendum": both are Latin-derived words whose claim to a "proper" classical plural is genuinely weak, since the classical Latin usage that would justify one essentially didn't exist for "virus," and was grammatically shaky for "referendum." Some writers in the early 20th century experimented with "viri" as a faux-Latin plural, mistakenly modeling it on real second-declension nouns like "cactus/cacti," but "viri" was never grammatically correct Latin and never gained real traction in English.

Usage note

"Viruses" is the only standard, correct plural in every context, from everyday illness to computer security and virology research; "viri" should be avoided as a mistaken, ungrammatical invention rather than a legitimate alternative.

The biological sense of "virus" wasn't established until the late 1800s, when scientists including Dmitri Ivanovsky and Martinus Beijerinck identified infectious agents smaller than any known bacteria, borrowing the old Latin word for "poison" to describe this newly discovered cause of disease. Computer science borrowed "virus" directly from biology in the 1980s to describe self-replicating malicious software, an entirely figurative extension based on the shared idea of something small and capable of spreading from host to host.

Common real mistakes

The main error here isn't a competing accepted form but a genuine mistake: writers occasionally invent "viri" believing it mirrors "cactus/cacti," without realizing that "virus" isn't even grammatically the same kind of Latin noun as "cactus," making "viri" simply incorrect rather than a valid stylistic register choice. Writers should also distinguish "virus" (a biological entity) from "bacteria" in casual health writing, since the two are entirely different types of microorganisms requiring different treatments, a factual distinction rather than a grammatical one.

The wider pattern

"Virus" and "referendum" both show what happens when a word's Latin credentials for a classical plural are weaker than they first appear — in "virus"'s case because the classical word was rarely if ever pluralized at all, leaving English free to build a fully regular plural with no real competing alternative.

Examples

• Researchers identified several new viruses in the sample.
• The lab tracks how quickly viruses mutate over time.
• Computer viruses can spread through infected email attachments.
• Several viruses were responsible for the seasonal outbreak.
• Vaccines are developed to target specific viruses.
• The outbreak involved at least two distinct viruses.
• Antivirus software scans files for known viruses.
• Scientists compared genetic sequences from dozens of viruses.
• Some viruses can remain dormant in the body for years.
• The report warned about emerging viruses spreading through livestock.
• Health officials tracked how the viruses spread between regions.
• The lab sequenced genomes from several newly identified viruses.
• The outbreak was traced to a single mutated virus.
• Public health teams monitored how the viruses spread through the city.
• Scientists raced to sequence the newly discovered viruses.
• The clinic offered free testing for common seasonal viruses.
• Farmers lost crops to a virus that spread through the region.
• The vaccine targets several strains of the circulating viruses.
• The lab isolated several viruses from the contaminated water.
• New treatments target viruses resistant to older medications.
• The outbreak involved viruses never previously seen in the region.
• The clinic reported a rise in seasonal viruses this winter.
• Researchers compared how different viruses evade the immune system.