Talking about the flu is different from talking about having a cold. One person can have a cold but the whole family may have an immunity to colds in general. That is to say, the plural of cold is colds just as the plural of virus is viruses. But the plural of flu is not flus.
Flu is short for influenza and for some inexplicable reason, the plural and singular form of this word are the same –
"I had the flu in October."
I like to avoid people with colds and flu."
You’re safe to use the word flu in the plural or you can say influenzas. This is one of those odd grammar rules that we just need to commit to memory, much like the Advil rule. I used to say that I took two Advil but once again the singular and the plural of that word are identical – whether you take one or 100 Advil, you don't want to add that “s” but I wouldn't suggest taking that many no matter how bad your flu is!
Sigrid Macdonald is a manuscript editor and the author of three books, including Be Your Own Editor. Buy now on Amazon Kindle for $2.99.