When your mom or dad gets sick, it's scary. You're used to having a parent take care of you. Suddenly your mom or dad needs to be taken care of instead. When that happens, everything seems backward. You're upset and confused, anxious and angry all at the same time.

Most kids don't have to worry about their parents' coming down with more than an occasional cold or a bad case of the flu. But serious illness does strike some families. What happens to children when their mothers or fathers get sick?

In her book, "When a Parent is Very Sick" (Little, Brown; $12.95), Eda LeShan writes about what kids go through during this difficult time. Mrs. LeShan is a teacher, writer and family counselor. She says that all children who must deal with an emergency, such as a parent's heart attack, treatment for cancer or injuries from a bad car accident, go through similar feelings.

At first, hearing the news about the sickness causes a feeling of shock and disbelief. "This can't be true," the children think. "It'll go away." Before long, children of sick parents start to feel scared. They wonder who will take care of them now. They worry that they will catch their parent's bad sickness, too.

Kids with sick parents often wonder if the disease is their fault. "If I hadn't been bad and made Dad worry, he wouldn't have had a heart attack," they may think. But it's not a child's fault when his mom or dad gets sick.

It's a good idea to talk all these feelings through with a teacher, a relative, a grown-up friend or the other parent. Serious illness is unfair and sad. When one parent gets sick, it may upset and distract the well parent so much that the children's feelings get ignored for a little while.

Kids with sick parents often get really mad, too. They may find themselves being mean to their sick mom or dad, just at the time that they most want to be nice to them. That anger is a normal response to the crisis.

When someone you love is sick, you feel helpless. Feeling out of control can make people feel really mad. Sometimes the best thing for them to do is to let the feelings out and do some crying. That often makes it easier to go on.

It's tough for a family to cope during the scary weeks surrounding a health crisis like a car accident or an operation.

It's also hard to live with a chronic illness -- one that the sick person will have for a long, long time or even for the rest of his or her life.

Kids whose parents have chronic illnesses often feel worried. "Worrying is a kind of feeling helpless -- you feel there is nothing you can do," Mrs. LeShan writes. It can help to do something instead of spending time thinking about the situation.

"The bad feelings seem to go away at least some of the time if you paint a picture for your mother's room . . . or pick some wildflowers and put them in a vase. Helping the well parent can also make you spend less time worrying," she writes.

Doing something is the way Matthew, an 11-year-old, copes with his dad's chronic illness. Matthew's dad has MS, or multiple sclerosis, a disease that affects the central nervous system. It interferes with the messages the brain sends to the muscles; it is not contagious, and children do not inherit it from their parents.

So far, doctors aren't sure what causes the disease, and there is very little they can do to treat it. But with careful medical care, many people with MS live completely normal lives. However, the disease can cause people who have it to feel very tired and weak, or to become paralyzed in parts of their bodies.

Some people, like Matthew's dad, have to use wheelchairs to get around. Matthew has appointed himself to be his dad's "ramp man." It's his job to wheel his father's chair up and down ramps.

Matthew also helps by reading legal papers out loud to his dad, who's a lawyer. He also helps out a lot around the house by doing laundry, making breakfast and taking care of the family's pet cat.

Matthew doesn't think that the extra chores he does and the extra attention he gives his dad are such a big deal. "If you love someone, you do what you have to do," he says.Tips for Parents

Experts agree that children sense the gravity of the situation when serious or life-threatening illness strikes. "Children know something is different with a parent even before the age of 2," says Dr. Minna Wolfe, a psychologist for the North Texas Chapter of the National Multiple Sclerosis Society. Dr. Wolfe's remarks appeared in "Inside MS," the society's quarterly newsletter. "Moreover, children tend to relate everything to themselves, so they may somehow feel that a parent's illness is their fault. It's essential to share with them and reassure them almost from the beginning. Keep explanations honest but simple, always reinforcing what you say with statements of security and love." For more information on MS, contact the National Multiple Sclerosis Society, 205 E. 42nd St., New York, N.Y. 10017; (212) 986-3240. Catherine O'Neill is a free-lance children's writer based in Baltimore.