Hygiene Lapse (mainly western)

Innoculation outbreak, Egypt infected 10% of it's population vaccinating against a Nile Mosquito. Some Third world schools have a 10% infection level, seemingly from the same route.

Mass Overuse (mainly third world)

Up to one in 70 people on the planet have HCV from syringe re-use.  Jet Guns and Needle Guns used to mass vaccination across the globe from 1965 to 1985 motored both Hepatitis B and C infections to unprecedented levels.