Rachel Hubbs, age 12, of White Plains, N.Y., for her question:

WHAT CAUSES ARTHRITIS?

Arthritis is an inflammation of the joints of the body. There are many different types of the disease. Some are caused by infections, some by injury, some by aging and some by entirely unknown causes.

Infectious arthritis may follow influenza, typhoid fever, tuberculosis, syphilis or gonorrhea. Arthritis of unknown cause is common. Rheumotoid arthritis is its worse form.

Chronic arthritis has three main forms: osteoarthritis, rheumatoid arthritis and gout.

Osteoarthritis comes about from wear and tear on the joints. Most people over 50 years of age have osteoarthritis in some degree. It is often called "old age rheumatism."

This type of arthritis is the most common cause of painful knee, backs and fingers. But it seldom causes crippling. This form is a painful nuisance to many persons, because it forces them to slow down.

The treatment of osteoarthritis consists mainly of moderation of activity, use of heat, taking aspirin and, in severe causes, injections of cortisone drugs into the painful joints and the use of braces to relieve strain.

Rheumatoid arthritis is often called the "great crippler." This form can develop in persons of any age but it most commonly attacks those in middle years. The disease is progressive, involving many joints. It causes much swelling and pain.

Treatment includes a well balanced diet, adequate rest balanced with enough physical exercise to prevent stiffening, the use of heat on the joints, and taking plenty of aspirin and other pain killing drugs. Doctors also often use injections.

Gout produces severe swelling in the joints. Crystals of uric acid are deposited in the tissues grounds the joints. These deposits cause sudden attacks of swelling, particularly in the feet. Attacks of swelling are called gouty arthritis.

Injections of gold salts and the use of cortisone drugs over long periods of time have helped certain persons troubled with chronic rheumatoid arthritis. Often this treatment prevents crippling.

In some cases of this disease, replacement of patients' joints with plastic or rubber joints enables them to move without pain.

Rheumatoid arthritis has no known cure, but more than half of the patients recover eventually.

There are about 112 million persons in the United States and Canada who suffer from some form of arthritis. The term is often applied to all the conditions that cause stiffness, swelling, soreness or pain in the joints.

Research projects are sponsored by the Arthritis Foundation, with headquarters at 1212 Avenue of the Americas, New York, New York 10036. The organization also publishes information on the prevention and treatment of arthritis.