A mysterious immune-system disease being investigated by the federal Centers for Disease Control in Atlanta continues to strike two people a day, on average, and has killed more than 200, while the hunt to find the agent that spreads it continues without success.

Fresh cases are now being reported to the CDC at the rate of two a day, said Dr. Harold Jaffe, a leader of the task force, and there are 505 confirmed cases of the disease with 202 dead.

The survivors continue to be susceptible to the many fatal illnesses that invade after the initial disease destroys the body's immune system.

Because a few hemophiliacs may have gotten the disease through blood concentrates obtained from blood banks, Edward Brandt, assistant secretary of Health and Human Services, has ordered a committee of experts to report to him soon on the implications for others using blood from banks.

Most Americans are not vulnerable to the disease now, and may not be at any time throughout the spread of the disease, because for some reason it has so far attacked only four groups of people.

About 80 percent of the those afflicted are homosexual men. Haitian immigrants and drug addicts who take their drugs through needles account for most of the rest who have the disease. In addition, a few sufferers of hemophilia have been identified among the victims.

The disease also has retained throughout its two-year recorded history a strong geographical bias. About 50 percent of its victims live in New York. Twenty percent live in San Francisco or Los Angeles. Because of the growing number of Haitians affected, Miami also accounts for a substantial number of cases.

In the District of Columbia, Maryland and Virginia eight cases have been identified.

Though the disease is limited geographically and to certain groups in the population, it is extremely deadly. It has killed more than twice as many as two previous emergencies--the outbreak of Legionnaire's disease in Philadelphia and the five-year run of toxic shock syndrome.

The epidemic is also unusual in that it has no known parallel in history. Doctors at the CDC say that no other disease has been recorded in which immune-system failure was spread from person to person.

Immune suppression is not in itself fatal, but when the body's defenses are destroyed other diseases can invade. The leading killers are a skin cancer called kaposi's sarcoma and pneumocystis pneumonia. Neither of those diseases is usually fatal, but both have taken a high toll because the victims of the immune-system disease may get the secondary diseases again and again, or several together. First notice of the epidemic came from doctors at New York University who saw an unusual number of the kaposi's sarcoma among homosexual patients.

Doctors in California then began reporting similar aberrations. The CDC and private researchers have worked for more than a year tracking the epidemic, taking detailed histories from the victims and trying to determine the common factors in victims.

They have also carried out hundreds of tests to try to discover whether the agent is a virus, bacteria, fungus, parasite or some common environmental condition. Though none of the virus tests has proved the point, doctors now say the best theory suggests that the agent is a virus that attacks the immune system.

It may be transmitted through the blood or other body fluids. For homosexuals, it is through semen. For drug addicts it is from blood-encrusted needles. For the few hemophiliacs, it may be from the blood concentrate that they must take daily, which is obtained from blood banks.