We had the Flea!

A few weeks ago Neal & Dustin (3 1/2 yr old) came down with the FLU BUG...fever, headache, body aches etc....Dustin came up to me this morning and said, "mommy, do you have the Flea too?"....I said, "Whats the Flea?" and he said, " you know, daddy says it's a bug that is in your body that makes you feel sick!".....I said....."Oh yeah!". I guess I could have corrected him and said that it was really call the Flu bug and not the Flea bug.....but why?