It seems to me that many, many parents still ship their kids off to school or day care while the kids are contagious, thus perpetuating the illnesses. A lot of times this is so the parents don't have to miss work.
In my neck of the woods, there are little-mentioned out breaks of meningitis almost every year. This year, scarlet fever has made a come back in two nearby towns. Scarlet fever is caused by strep A and can turn into rheumatic fever or worse if untreated. Late last year it was whooping cough showing up. My point is, that these are all naturally occurring.