When Tom woke up the other morning, he knew right away that something was the matter with his throat. When he tried to swallow, his throat hurt. He tried again. OW!

"Mom," he called. "I have a sore throat." When he yelled, his voice sounded sort of scratchy and funny.

"Not again," his mom said. (Tom's mom isn't unsympathetic. It's just that he's had quite a few sore throats this winter.)

Why does Tom have a sore throat? He might have caught a cold. He might have the flu. Or he might have strep throat. Sore throats also go along with measles, chicken pox, whooping cough and tonsillitis.

"A sore throat is often the first symptom of a cold," says Leonard Rybak, a doctor at Southern Illinois University in Springfield who specializes in illnesses affecting the ears, nose and throat. "Viral and bacterial infections cause most sore throats," he adds. Colds and flu are viral infections; strep is an infection caused by a germ called a beta-streptococcus.

If Tom has a cold, his mom won't be surprised. As you probably know from your own experience, kids get lots of colds in the winter. Doctors confirm this. Kids in nursery school may catch as many as 12 colds a year. Older kids average between six and nine colds a year. Most kids have caught more than 100 colds by the time they're 18! Luckily, by the time you grow up, you average only two colds a year. That's something to look forward to.

For most sore throats caused by viruses, doctors say, the body is its own best healer. It goes to work to get rid of the invading viruses, a process that usually takes about a week. Sucking on soothing lozenges or gargling with warm, weak salt water can help. Dry indoor air can irritate a sore throat, so many people turn on humidifiers when they have colds or flu.

Medicines called antibiotics can quickly knock out a strep infection -- but first you have to have a test done to see if you have strep bacteria in your body. A doctor can do this quickly by studying a sample of the mucus in your throat. This doesn't hurt, and it only takes a few seconds. The doctor uses the sample to do a throat culture.

But why does your throat hurt when these illnesses come along? It's not like something hit you or cut you in there.

To understand what happens when someone gets a sore throat, let's take a look at that part of the body and what it does.

The throat has three vital jobs. It carries food from the mouth to the stomach. It carries air to the lungs. And it makes it possible for us to speak. With all that work to do, it's not surprising that the throat gets sore from time to time.

A moist lining called a mucous membrane lines the throat. When you get a viral or a bacterial infection, this lining swells up and gets irritated. Then your throat feels scratchy and sore. If your nose is dripping, too, the mucus from your nose drips down into your throat, causing even more irritation. The throat gets red and raw. It hurts to swallow.

A cough often goes with a sore throat. The cough is actually a good sign: It means you're getting rid of extra mucus that builds up as your body dispels invading cold or flu germs.

Tom's voice is temporarily hoarse because whatever is causing his sore throat has also irritated his vocal cords. These cords are structures in your throat that vibrate as air passes them, making the sounds we call speech. The vocal cords also help prevent food and water from entering your lungs when you swallow.

Tom kind of likes being hoarse; he thinks it makes him sound older, but the change is only temporary. Once the inflammation in Tom's throat clears up, his voice will go back to normal again.

Tom's mom takes his temperature. "No fever," she says.

"AAAAAAA-choo!" Tom answers.

"Sounds like you have a cold," Tom's mother says. "Better stay in bed today. I'll bring you some ginger-ale and some tissues. And I'll call your teacher to make sure you get your homework assignments."

Tom sneezes again. Then he reaches for the comic book on his bedside table. Having another cold isn't all that bad, he thinks.

Tips for Parents

Should you call a doctor if your child complains of a sore throat? The American Academy of Otolaryngology-Head and Neck Surgery recommends that children -- and adults for that matter -- need medical attention if:

A sore throat is accompanied by difficulty breathing, swallowing or opening the mouth.

A sore throat is severe, prolonged or recurrent.

A sore throat is accompanied by joint pain, earache or a lump in the neck.

If there is a rash or fever over 101 degrees.

If hoarseness lasts more than two weeks.

For more information, send a self-addressed, stamped business-size envelope to: "Sore Throats," American Academy of Otolaryngology-Head and Neck Surgery,

1 Prince St., Alexandria, Va. 22314.

Another tip: The American Academy of Pediatrics recommends against giving children cold and cough remedies that contain more than 5 percent alcohol. Such syrups must list ingredients, so check before you buy.

Catherine O'Neill is a children's writer.