However, you do not need to worry constantly, since babies only get sick at certain times. For instance, babies never get sick while they're at the doctor's. They only get sick at night. Late at night. Fortunately, you can always tell when the baby is sick.
Babies vomit when they've had too much to eat.
They vomit when they play too roughly. They vomit when they're teething. They vomit when they have ear infections. And sometimes they vomit just for the sheer joy of it. The best way to keep babies hydrated is to supply clear liquids.