Sheila Canfield, age 15, of Denton, Tex., for her question:

WHAT CAUSES MONONUCLEOSIS?

Mononucleosis is an acute infectious disease that causes fever, sore throat, extreme tiredness and enlarged lymph nodes. The organism that causes the disease is unknown, although most medical experts believe that it is a virus.

Sometimes called the "kissing disease," mononucleosis can be spread by kissing, but also by coughing and sneezing. Mononucleosis is found in almost every part of the world.

Usually it strikes people between the ages of 10 and 35. The first symptoms are extreme tiredness and a headache. The tiredness does not go away after the first long sleep, and by then the victim usually has a fever and sore throat. The fever generally is between 100 and 103 degrees Fahrenheit. Red spots on the roof of the mouth often follow on about the seventh day of the illness.

Another symptom is enlarged lymph nodes, most frequently those in the neck. Lymph nodes in the armpits, groin and abdomen may also swell.

In about half the cases, the spleen also enlarges. Occasionally it may rupture, requiring surgery. Small skin lesions, pinkish in color, sometimes form a rash over the arms and chest.

The victim's white blood cells increase. Lymphocytes are called mono nuclear cells because they have only a single nucleus. Doctors say that the most beneficial therapy for mononucleosis is four weeks or so of bed rest, because general treatments for the disease are limited. Aspirin and other pain medication along with a steroid drug called prednisone are sometimes prescribed.

The outlook for a person with the disease is good.

If a mononucleosis victim's spleen ruptures, emergency surgery is required. In rare cases a rupture can cause death. Mononucleosis can continue for as long as two or three months. The fever, however, usually disappears after 10 days.  Sometimes hepatitis occurs as well. In a majority of the patients, this condition will subside after about five weeks.

Mononucleosis may also cause a rapid pulse and irregular heartbeat. Nervous system involvement is not uncommon for victims of mononucleosis.