(Foil Hat)

IIRC until about 1999 there wasn't a vaccine for it, and if you get it as an adult you go completely sterile. AFAIK parents would actually hold "pox parties" to get children infected to build up immunity. I had it in the mid-90s (see the whole vaccine thing). Judging by Equestria's apparent technology level (They have movies but the only TV screen we've seen so far is on an arcade machine), vaccines or no there most likely wouldn't be one for this one, especially if it is chicken pox…