Jack Maynard, age 12, of Wilmington, N.C., for his question:

WHAT CAUSES RHEUMATIC FEVER?

Rheumatic fever is caused by bacteria that belongs to the Streptococcus group, the same group that causes blood poisoning and scarlet fever. It is often a disease of young persons.

Rheumatic fever may damage the heart valves so that blood cannot flow through them normally. As a result, the heart must work harder to keep up with the needs of the body.

No one knows exactly how the rheumatic fever bacteria attacks the body.

The disease usually starts with vague, mild pains in the muscles. Soon the pains become intense and the person's joints swell. The disease may subside in a few weeks or may continue for many months or even years.

Rheumatic fever is one of the greatest causes of heart diseases in young persons.