Two rare diseases have suddenly struck more than 100 homosexual men in the United States and killed nearly half of them, in a medical mystery that appears to be on the scale of the toxic shock syndrome or Legionnaire's disease.

The Centers for Disease Control in Atlanta has found 70 new cases of the two linked diseases since July 3, when the CDC reported 40 cases, then thought to be a very high number.

A force of 20 investigators from the Centers for Disease Control has begun hunting the cause of the sudden spread of the diseases, a skin cancer and a form of pneumonia. They have been reported in 111 patients in recent months; at least 43 patients, and possibly several more, have died from one or the other.

For comparison, the toxic shock syndrome, found in now decreasing numbers, killed 84 women during more than two years of reporting.

The number of cases of the two new diseases reported among homosexual men is up to 200 times the expected rate, said Dr. James Curran, chief of the CDC task force.

Both diseases are rare, and neither is usually fatal. The strain of pneumonia called pneumocystis is usually found only when other diseases have hit the system first, making it more vulnerable to infection. The cancer, called Kaposi's sarcoma, is usually found only in old men, also with impaired immune systems.

In the current outbreak none of the usual rules applies. The diseases have struck far more frequently than is normal. They are not frequently fatal, but have killed 40 percent of those infected. The men were healthy and so should not have got the pneumonia. And they were young, and so should not have gotten the skin cancer.

"Really, there is no logical, obvious cause," said Curran. "There is something wrong with every one of our theories."

The diseases sometimes have appeared separately, and sometimes in combination.

The CDC investigators have two clues -- that both diseases are striking only homosexual men, and that both normally attack people who have badly damaged immune defenses, for example, kidney-transplant patients taking drugs to suppress the body's defenses against newly implanted tissue.

"It may be that both are piggybacking on the severe breakdown of the immune system in these men," said Curran. But why only men? Why only homosexuals? And why in healthy men who had no apparent challenges to their immune systems?

Curran said they are investigating the chance that the men may have taken some drug contaminated with a chemical that would attack the immune system. They are also investigating causes connected to the ability of some viruses to attack the immune system.

Neither of these explains why the normally mild diseases now appear in such a deadly form.

One suspected agent, direct or indirect, is a herpes virus called cytomegalovirus, which is present in a dormant form in half the population and in as many as 95 percent of homosexuals. When active, the virus is present in body fluids such as blood and semen, and may be passed by direct contact from person to person. The virus has also been found in some Kaposi tumors.

The CDC team is interviewing patients and a control group of homosexuals in several large cities, seeking factors common to those stricken and absent from those not taken ill. Preliminary answers are expected to take several months to find.