The most common cause of vomiting in babies is an infection of the intestinal tract by any virus that happens to be going around. The illness usually starts with a sudden bout of vomiting, often with fever or diarrhea (not necessarily in that order). Most infections run their course in two or three days, although a child's tummy often isn't up to snuff for days after.
The biggest risk involved with these viruses is dehydration. Red flags that your baby is getting dehydrated are a drop-off in the normal number of wet diapers and a shortage of saliva. If your baby won't take -- or can't keep down -- breast milk or formula, offer her a tablespoonful of an electrolyte solution such as Pedialyte or Rehydralyte every 15 minutes or so. Call your pediatrician if she's vomiting up the solution.
In rare instances, vomiting in infancy can indicate that baby was born with, or has developed, a malformation of the digestive tract. One common condition is called pyloric stenosis, which occurs when the muscle at the exit of the stomach thickens, preventing milk from passing through it. No one knows what causes it, but it usually shows up in babies between 3 and 5 weeks of age. The telltale sign of pyloric stenosis is projectile vomiting -- vomiting that's forceful enough to shoot across the room. If your baby is diagnosed with pyloric stenosis, usually by an ultrasound, he'll need an operation to open the blockage at the outlet of his stomach.