FMLA is validly used for a "serious health condition" of either the employee him/herself or of a family member. "Serious health condition" is defined as illness or injury that, among other things, lasts for over three days and requires repeated medical treatment. There's more to the definition; you can find it on the DOL web site if you want to look for it.
Chicken pox can in some cases be quite serious and back trouble can indeed be disabling. I'm by no means saying that there is no FMLA abuse going on at your company; it's quite possible that there is. There's a fair amount of FMLA abuse going on everywhere. But when something falls in the grey area, it's wise for the employer to to err on the side of caution, since FMLA discrimination claims tend to be settled in favor of the employee. That's how this can happen.
