Grace, a 10-year-old, was getting ready to go off on her vacation when she developed a bad stomachache. It felt different from other stomachaches she'd had before.

Grace's stomachache started as pain in the area of her belly button. Later it moved down to her lower right side, above the hip bone. It really hurt -- especially when she moved or coughed. Gracie told her parents about the pain, and they took her to the doctor right away.

Gracie's doctor checked her symptoms. She felt sick to her stomach. She had also developed a high fever. The doctor decided Gracie had appendicitis.

When they heard the news, her parents decided to postpone the family's vacation. Gracie was admitted to the hospital for an appendectomy. After her operation, when she was feeling completely well again, they set off for the beach at last. Gracie felt like she'd really earned her vacation.

When you get a bad stomachache, do you ever wonder if you have appendicitis? Many people, both grown-ups and kids, do worry about it -- and for good reason. Appendicitis can be a pretty serious illness unless it is treated quickly.

What is an appendix, anyway? Its name gives you a clue. The medical name for this body part is the vermiform appendix, or "worm-shaped attachment." Appendix means "added on" or "attached to." That's a pretty accurate description. Your appendix is a little dead-end tube or sac about three inches long that dangles from the lowest loop of part of your large intestine. It's ordinarily located on the lower right side, though in some people it's on the other side.

The appendix doesn't seem to have a job to do in the body -- it's just an extra piece of equipment. But while human beings apparently don't need the appendix, some animals do. In rabbits, cows and some other plant-eating animals, the appendix helps break down the tough fibers in the leaves and grass they eat. Some people think that primitive human beings, long ago, might have had much larger appendixes than we have today, because they ate more fibrous plant material. Then the piece of extra intestine might have been useful. But no longer.

In humans today, the only thing the appendix does is occasionally cause trouble. Sometimes a small, hard particle of undigested food may drop into the hollow inside of your appendix on its way through your intestine. If the particle gets stuck, it may block up the entrance to the appendix. Bacteria -- tiny organisms that normally live in your intestine -- get trapped inside. Under normal conditions, bacteria help break down the food your eat. But when they're trapped inside the appendix, they start to cause trouble. They continue to do what bacteria do best: multiply. The number of bacteria inside the appendix gets larger and larger. This irritates the appendix, causing it to swell up, turn hot and red, and hurt. This condition is the medical problem we call appendicitis.

If the condition gets serious, then surgery is necessary. A doctor has to remove the appendix before it bursts, or ruptures. If the appendix bursts inside the patient, a serious infection may spread through the abdomen.

An English doctor performed the first appendectomy in 1736 -- more than two centuries ago. Today, doctors are still performing a similar operation -- although they use much more modern methods. Most people who undergo appendectomies are between the ages of 5 and 30. Appendicitis most commonly occurs between the ages of 15 and 24, though doctors aren't sure why. In 1983, 282,000 people had the operation in the United States. Tips for Parents

Dr. David Stewart, a pediatric surgeon at Georgetown University Hospital, calls appendicitis a protean disease, meaning it can behave in highly variable ways and be difficult to diagnose. Parents should be aware of a trio of symptoms -- pain, nausea and fever -- but should keep in mind that not all of the symptoms may be present at once. A classic case of appendicitis begins with abdominal pain in the region of the navel and moves into the lower right quadrant, above the hipbone. Stiff, rigid abdominal muscles may accompany the pain. Pain is generally followed by nausea and vomiting, and an elevated temperature of around 101 degrees. If the temperature is much higher, a rupture may have already occurred.

If your child says he's feeling severe pain in the abdomen, take his complaint seriously. Consult a physician as soon as possible. Appendicitis caught and treated early involves a quicker recovery from surgery. Often, says Stewart, a child awakens from the appendectomy feeling better than he did when he came into the hospital.