The flu or just flu?

You may be sick of questions on articles, but...

"Flu is a severe infection caused by a virus."

"The flu is an illness caused by the influenza virus..."

I found both sentences on the net. Is the definite article before "flu" optional, or is there a rule of some sort as to when to put a definite article?

I can understand that, for example, you now say "The flu is going around..." since the new corona virus is rampant these days and here you are referring to it (hence "the"). But in such a case as I wrote/quote above, like giving a definition, what's the rule?

Thank you,

Klingo