When your grandparents were little, there was a word that made people shiver when they heard it. That word was polio. Polio is short for a sickness called poliomyelitis. Mild cases caused flu symptoms: fever, headache, sore throat. But bad cases of polio could damage nerves and paralyze muscles.

Polio often affected leg muscles, so patients couldn't walk any more, or chest muscles, so the patient couldn't breathe. Machines called iron lungs were developed to help them breathe. But many polio patients died.

Sometimes, the paralysis improved with time and treatment. But polio left many people -- especially children -- crippled for life. Polio is contagious. That means that it can be passed from one person to another. The virus that causes polio travels in the body's waste products, from where it may be spread directly or indirectly on fingers to food to infect others. Airborne transmission also occurs. But when your parents were growing up, people weren't sure how polio was transmitted. They thought you could get it just by being near someone who had it.

The terrifying threat of polio ruined many summer holidays. Pools and beaches were closed. People were told to avoid crowds. Movies were canceled. People didn't go to parks. Ask your grandparents or your parents if they remember being scared of catching polio; they'll probably say yes. During the first half of this century, there were repeated outbreaks of polio in the United States. Once an outbreak started, it spread quickly. Many cases developed in a town, a city or other populated area. Doctors have a special name for an outbreak of disease that spreads quickly through the population: epidemic.

Polio epidemics became especially bad during the 1950s. At the beginning of the '50s, there were about 30,000 new cases of polio reported each year, and the disease was becoming more and more common. By 1955, there were 58,000 new cases. Parents really started to panic. Kids were hardly even allowed to play outside in some parts of the country.

But then there was an amazing breakthrough. Medical science stopped polio dead in its tracks. Researchers developed a medicine called a vaccine that could prevent people from catching polio. In 1954, Dr. Jonas E. Salk introduced the first polio vaccine. Another form, developed by Dr. Albert B. Sabin, came along in 1961. Soon, all children were given polio shots. (By the way, you received the polio vaccine when you were little, too.)

By 1963, polio claimed only 400 new lives a year. By the 1970s, polio was practically unknown. The epidemics were over.

Measles is another word that used to scare parents. Many people -- both kids and adults -- used to die from this illness. Measles is one of the most contagious illneses around. Its germs pass from person to person when they touch each other or in the droplets from sneezes and coughs. Doctors developed a vaccine to prevent measles, too. It was first used in the U.S. in 1963. The vaccine quickly reduced the rate of measles infection in the country. In 1962, there were 482,000 cases of measles, but by 1968, there were only 22,000.

But measles has not disappeared, because not all children have been vaccinated. In 1990, measles has broken out in 25, states and some children have died. They had not received measles shots. This year, doctors urge parents to protect children by getting them a booster shot.

Both polio and measles are caused by invisible germs called viruses, which invade and live in the cells of other living things. Once inside the body, viruses multiply like mad, causing sickness. Some sicknesses, such as polio and measles, caused by viruses can be prevented by getting vaccinated. Others, like colds or the flu, are killed by the body's own germ-fighting ability. This defense system is called the immune system.

Some illnesses caused by viruses can't be prevented or cured -- yet. The disease called AIDS is caused by a virus. AIDS weakens the body's ability to fight infection, so people with AIDS catch many different illnesses much more easily than healthy people do.

To us, the word "AIDS" can sound as scary as "polio" sounded to our grandparents. The AIDS virus is not easy to catch, and very few children get it. AIDS is transmitted when drug addicts share needles or when someone who has the disease has sexual intercourse with another person. Some babies are born with AIDS because their mothers already have the disease.

You can't get AIDS from touching, hugging or spending time with someone who has it. You can't get it from breathing the same air they breathe or by holding hands with them.

Scientists have developed some medicines that can ease the symptoms of AIDS or slow the disease down. But there is no vaccine to protect us from AIDS. There is no cure for AIDS, either, so people who have it eventually die.

In hospitals and labs all over the world, doctors and researchers are working hard to find medicines that will help. When you feel scared about AIDS, remember that other viruses have been beaten by medical science. Everyone hopes that the AIDS virus will be beaten, too.Tips for Parents

An excellent book on AIDS is "Children and the AIDS Virus: A Book for Children, Parents and Teachers" by Rosmarie Hausherr (Clarion Books; $4.95). The book has two texts: a simple one for young readers and a more complex one for parents that will enable them to answer questions. The books also provides extensive references for finding more information. Part of the money from the sale of the book goes to the Children's Immunology Research Fund, Inc. Catherine O'Neill is a freelance children's writer.