It can happen from eating spoiled food, which they won't do on their own, and which I won't force them to do. It can also happen from having refined Tiberium in their personal inventories for more than a day, which causes radiation poisoning (it's not fatal, fortunately). Both are preventable causes. The third cause of vomiting only happens to female Sims and it isn't avoidable; it's called "morning sickness". 
