What is the definition of Ahead Infection?

An "ahead infection" is like when the germs decide to sneak up on you before you even realize it's happening. Imagine this: "After eating that sushi, I caught a 'head-on' infection from the fishy fungus, but I didn't realize it was an 'ahead' thing until I saw my reflection in a puddle - because apparently, the mirror knows when the bad guys are coming!"