That doesn't mean much. The dictionaries in these browsers are woefully inadequate. They miss pretty common words all the time. Yes, ones which I know are spelled correctly, because I'll be so shocked that it isn't recognized that I'll research it even though I'm certain of the spelling. I don't know why they're missing so many words.

He specifically said the Latin plural, though, so maybe the spellcheckers miss that. The English form appears to be viruses...