Grammar. Plural forms of nouns

 Most singular nouns form the plural by adding -s. Examples: a cat - many cats; a box - many boxes, a baby - many babies, potato – potatoes.

There are some irregular noun plurals. Examples: a means - many means, species – species, mouse – mice, focus – foci, cactus – cacti, fungus – fungi, nucleus – nuclei, analysis – analyses, diagnosis – diagnoses, thesis – theses, crisis – crises, phenomenon – phenomena, criterion – criteria, datum – data, bacterium –bacteria. 

But: virus – viruses!