SNIFF! The boy in the seat next to you at school has a cold. ACHOO! He sneezes loudly -- and he doesn't remember to cover his mouth with a handkerchief. He rubs his nose with his hands -- and later passes you a pencil you dropped.

When your classmate sneezed, he probably sent his cold germs flying all through the room. When he touched your hands, he may have passed some of his germs along to you.

There's a chance you might catch a cold, too. Most kids catch from two to six colds a year. Colds are the most frequent form of illness in children, and in grown-ups, too -- although adults get fewer colds.

Scientists have found cures for many of the illnesses that people catch, but they still haven't discovered a way to cure the common cold.

Colds are caused by viruses -- tiny, invisible germs. More than 200 different cold viruses have been found. That's a lot of different ways to get the sniffles and sneezes. If you catch one kind, and get over it, you can still catch another kind. That's why you may catch several colds every year.

Luckily, colds aren't usually very serious. You may feel, look and sound miserable for about a week before you get better.

Colds cause the inside of your respiratory tract -- the part of you that does your breathing -- to swell. The wet lining of the tract, called the mucous membrane, becomes wetter than usual. Your nose starts to run.

The insides of your nose, mouth, throat and ear passages are all connected. You may be especially aware of this when you have a cold. You get a runny nose. Other fluids run down your throat, and along with the virus, make it scratchy and sore, and sometimes bringing a cough along. Your ears may feel funny, too.

You may have heard that taking a lot of vitamin C can prevent colds. Some doctors think this works, but others disagree.

People catch the largest number of colds between September and May. That's when you spend the most time indoors. The heating system in your home or school dries out the air -- and dries out your mucous membranes. That may make you more likely to catch a cold. Being around people who don't wash their hands frequently when they're sniffling and sneezing increases your chances of catching cold, too.

Getting over-tired, being very, very busy, or eating a poor diet may make you more likely to get sick. But even if you sleep well, stay happy, exercise regularly and eat a balanced diet, you probably will still catch at least one cold during the school year.

What can you do when that happens? Not much.

Americans spend half a billion dollars every year on cold medicines. These medicines may help the symptoms of your cold, like sneezing and coughing. But they don't cure the sickness.

When your grandmother was a little girl, her mother probably made her rest and drink lots of liquids -- like juice and soup -- when she caught a cold. Today, those old home remedies are still a good idea.

When you blow your nose, you lose fluid from your body, which needs to be replaced. Just as your parents make sure there is enough water in the radiator of the car to keep it running smoothly, you need to drink enough water to keep your body's engine running smoothly, too.

So if you do catch a cold, remember to drink liquids and get lots of rest -- and you can look forward to getting better soon. Tips for Parents

Dr. Catherine DeAngelis, the deputy director of pediatrics at Johns Hopkins Hospital in Baltimore, usually tells children with colds that they can go to school if they do not have a fever -- but that they need to be careful not to spread the cold around.

Tell children, she says, to "cover your nose and wash your hands, and make sure to carry plenty of tissues."

DeAngelis offers a recipe for cold-combating hot toddies: 8 oz. of tea, 1 teaspoon of honey, and the juice from 1 wedge of lemon. The old home remedies, DeAngelis suggests, still do about as much for the miseries of a cold as anything you can get across the counter at the drugstore.

Since colds are viruses, they are not susceptible to treatment with antibiotics, DeAngelis says. "You don't need to get a penicillin shot."

All medicines have some side effects, she adds, so she discourages patients from taking over-the-counter cold preparations unless they feel "really terrible."

"I tell my patients to try not to take medicines because they may make them excited and jumpy, or sleepy and drowsy," she says. Read labels, she adds, because many cold remedies contain alcohol and shouldn't be given to children.

On the bed-rest issue, DeAngelis says: "Parents may need to have a cranky, coldy child spend some time in bed. But a child with a cold should be allowed to do pretty much what he feels like . . . The viruses that cause colds are pretty innocuous -- they might keep you out of work or school for a day or two, but that's about it."