Watching your infant vomiting is heart wrenching. Vomiting isn't fun, no matter what age you are. But there's something just so wrong about seeing that tiny body convulse to throw up.
This is where you step in, Mom, and take charge. And this is what you do.
First of all, understand a few basics about your vomiting baby.
Vomiting doesn't necessarily come packaged with diarrhea (a common misconception).
Despite the obvious happy reasons (who wants runny poopy diapers?), having a vomiting infant that doesn't have diarrhea is a major health win.
The more fluids flowing out of your infant's baby, the higher his chances of becoming dangerously dehydrated.
Also, learning the vomit/spit-up difference is an important step in choosing your path of treatment. Both have different causes, so both have different treatments.
Treating one when it's really the other will give you a great big pile of nothing. Decide on the problem first. Then you'll know how to best manage it.

Most infant vomiting episodes are caused by viral infections caused gastroenteritis. And yes, it is contagious. Start disinfecting the house.
Occasionally you'll find infant vomiting caused by a high fever. Obviously this type would not be contagious, since it's caused by the fever.
Other rarer causes include food poisoning, a stomach disorder, or meningitis.
When your baby vomits, try to keep her laying on her stomach or on her side as much as possible. This will prevent her from possibly inhaling the vomit into her lungs.

If he threw up once or twice, but otherwise seems in good spirits, simply keep a sharp eye on him.
Nurse (or bottle feed) him for a few minutes, and see how he handles the liquid.
Worst case scenario? He vomits it all up again.
Your greatest risk in infant vomiting is dehydration.
Breastfed babies usually tolerate breast milk well, even when vomiting. Offer just one breast, or smaller volume to test out his stomach. If breastmilk doesn't seem to be working, try the bottlefed solution below.
Bottlefed babies should be given, instead of formula) an oral rehydration solution (ORS) like Pedialyte
. I try to have some on hand for these unexpected occasions.
Use a bottle, spoon, or even a dropper to give the Pedialyte. This chart will give you an idea of how much (in 24 hours) he should receive.
Do not give your infant fruit juices, carbonated sodas (7-Up or Sprite) or sports drinks (Gatorade).

It's not the vomiting that's dangerous.
It's the dehydration.
The puke flies out like a rocket, spewing what seems like gallons of vomit everywhere. (Really, it's just the contents of the last bottle.)
Because of this explosive nature, it should be watched more closely.
If your infant shows signs of being lethargic, running a fever, or has diarrhea, a doctor call should be made.
Specifically, infant projectile vomiting is when the the stomach and diaphragm muscles to contract so forcefully, the vomit is projected several inches out of the mouth like a spray.
The fancy-pants doctor name for the most common cause of infant projectile vomiting is pyloric stenosis. Besides the obvious fire-hose spray sign, other signs of ploric stenosis are a lack of weight gain and a general look of malnourishment because no nutrients are being absorbed.
Another good clue that pyloric stenosis is the cause, is the regular pattern. It will happen about 15 to 30 minutes after every meal.
Pyloric stenosis occurs most often in firstborn boys (don't ask me why, not a clue) between the ages of 2 weeks and 4 months. It requires surgery to correct.

Usually infant vomiting goes away on its own, within 6-24 hours. However, if it's been going on longer than 12 hours or has been persistantly occuring with diarrhea, you should call your doctor.
You should also call the doctor if you see vomit that is green-colored, blood-tinged, or looks like coffee grounds. The same is true if his tummy looks swollen (distended) or if he continues to refuse to eat or drink.
He's going to be concerned about dehydration, so it would be a good idea to know what the signs are so you can anticipate his questions.
I would also recommend calling your doctor if infant projectile vomiting occurs. No matter what you feel may be the cause, this kind of forceful infant vomiting should be at least reported to your pediatrician for more handling guidance.

There's a very good chance you're going to open up a diaper sometime in the first year and be surprised at the green watery mess splattered inside. Diarrhea is very common in babies, and although treating it is really more about enduring than prescribing, without proper managemement it can quickly get very serious...
Does the thought of handling an infant fever leave you in cold sweats? That little plastic beeping stick can greatly reduce your mothering anxiety, yes. But it's only the first grade. You, my dear, need to graduate. You need a crash-course in effective fever manipulation.
Sometime's it's hard to tell the difference between a newborn who's vomiting, and one that's merely spitting up. Here's a clear definition of what to look for, and how to treat both conditions.