It's an ordinary day at Katie's school today -- just another winter Wednesday. But wait a minute. The final bell has already rung, but many of the desks in her homeroom are empty. What's going on? Is there an outbreak of hooky-playing going around?

No. The kids who usually sit in those empty desk aren't cutting school. They're home in bed nursing their scratchy throats and achy muscles. They're victims of this winter's outbreak of influenza -- better known as the flu.

Experts at the Centers for Disease Control in Atlanta, Ga., say that this year's outbreak of flu is the largest in five years. The Washington area -- including Katie's elementary school -- has been hit hard. This winter, many schools in the District, Maryland and Virginia have reported that one out of every three kids has been home with the illness. Other states have had flu outbreaks, too.

This morning, Katie's teacher looked out at her classroom. Seeing all the empty desks, she sighed and said, "I guess we'll skip the science lesson I had planned. We'd just have to do it over again when all the absentees get back."

Instead, the teacher decided to explain what influenza is. "Influenza is caused by a virus," she said. "Viruses are so small that they're invisible to the human eye and even most microscopes. But they're big enough to cause a lot of trouble."

Katie's teacher drew a picture of a cell on the blackboard. Then she drew a little dot that stood for a virus. She explained that a virus invades the larger cell, and changes the way it works. In fact, the virus turns the cell into a factory. What does the factory make? You've got it. More viruses.

There are many, many kinds of viruses. Luckily, only a few of them cause the flu. If you are exposed to a flu virus, it may start setting up flu factories in your body. Your immune system -- the part of your body that fights off infections -- quickly goes to work to fight the virus. But the battle can make you feel pretty awful while it's going on. Your throat feels dry and scratchy; your head aches. Your muscles feel sore. You feel weak, and you need to sleep a lot. Your fever may get pretty high. Some victims of this year's flu victims have had fevers as high as 104 degrees.

Once you catch the flu, what can you do? Unfortunately, not much. For some kinds of sickness, like ear infections, your doctor may treat you with antibiotics. These drugs destroy the bacteria, or germs, that are causing the problem. But scientists haven't found drugs that are safe to use and effective against most viruses yet.

Katie remembers that her grandmother got a flu shot last fall. If there isn't a good medicine for treating flu, how come people get the shots?

"Flu shots are vaccines," Katie's teacher explains. "The vaccines are made from a very mild form of flu virus. They make the body develop an immunity to that type of virus. When you're immune to something, you can't catch it."

Most doctors don't think children should get flu shots unless they're in a "high-risk" group -- a group that has other illnesses or conditions that could make getting the flu very dangerous. These high-risk groups include children who have heart, lung or kidney disease, or diabetes. Elderly people, like Katie's grandmother, get the shot because flu can be more serious for them.

Katie's grandmother had a flu shot, but she got the flu anyway. That's because flu viruses are tricky. They change, or mutate. In different years, slightly different types of the virus may appear. This year's flu is mostly caused by the "B" type, according to doctors at the Centers for Disease Control. People who got the vaccine in the fall may not be immune to Type B. And once you catch the flu, it's too late to do much about it except rest in bed, drink plenty of fluids, and wait to get better in a few days to a week.

At school, the bell rings for the next class. It's time for art, Katie's favorite subject. But she doesn't feel much like going to art class today. In fact, she feels a little funny. Her throat is scratchy, and she feels kind of hot . . .

"Uh-oh, I think I've turned into a virus factory," she tells her teacher. Tips for Parents

This year's flu outbreak has public health officials concerned about a possible upswing in the incidence of Reye's syndrome, a rare but serious condition that afflicts children and teen-agers and is linked to aspirin use.

A statement issued jointly by the Food and Drug Administration and the Health and Human Services Department warns parents not to treat their children's flu symptoms with aspirin. Aspirin substitutes, such as Tylenol and other acetaminophen drugs, are not implicated in Reye's syndrome.

The symptoms of Reye's syndrome usually appear just as the child or adolescent appears to be recovering from the flu. They are:

*Persistent vomiting;

Belligerent behavior;

*Lethargy progressing to coma.

Should such symptoms occur, early hospitalization can be lifesaving, the government health experts said.