Federal epidemiologists investigating a serious disorder of the body's immune system that has mostly afflicted male homosexuals reported new evidence yesterday suggesting that the outbreak is linked to an infectious agent.

''We think the findings are important but they don't solve the problem,'' Dr. Harold W. Jaffe, one of the epidemiologists at the Centers for Disease Control in Atlanta, said in an interview. ''They do show pretty convincingly that this is not occurring as a random event among homosexual men.''

No specific infectious agent has been identified, he said, but scientists at the Atlanta facilities are intensifying laboratory efforts to identify a virus, bacteria or other micro-organism as a possible cause.

The immune system breakdown has been implicated in a rare form of cancer, called Kaposi's sarcoma, and seems to lead to a wide variety of serious disorders. Scientists have designated the immune disorder GRID, for ''gay-related immunodeficiency disease.'' They Suspected Infectious Agent

From the time the disorder came to public attention a year ago, epidemiologists have suspected that it might be attributable to an infectious agent, although they have investigated several alternative hypotheses.

The latest clue to an infectious origin came from an intensive medical investigation of a cluster of 19 cases in Los Angeles and Orange Counties in California. It was the first detailed study of such a cluster in the syndrome.

Investigating epidemiologists from the Atlanta center and those in California said that ''within five years of the onset of symptoms, nine patients had had sexual contact with other patients'' with the syndrome.

They made that determination by interviewing the eight survivors among the 19 cases as well as friends of seven of the 11 who died. From those 15 cases, the epidemiologists obtained data on sexual partners for 13.

Of the nine people who had had exposure to other cases, they found that four had had contact with more than one case. Links Are Interconnected

Further, they said, ''The nine patients from Los Angeles and Orange Counties directly linked to other patients are part of an interconnected series of cases that may include 16 additional patients from eight other cities.''

Epidemiologists are planning to conduct further study of the links to the cases outside southern California and to seek similar clusters elsewhere.

''The more cases you put together in a diagramatic fashion, the more that supports an infectious agent, although it certainly does not exclude other possibilities,'' Dr. Jaffe said. ''We would like to see how far we can go pursuing connections outside of southern California.''

Scientists are using ''as many ways as we can think of to identify'' the infectious agent in the laboratory, he said.However, the epidemiologists said in their weekly report that they were still considering alternative hypotheses. One, they said, is that sexual contact with patients with GRID syndrome does not lead directly to the breakdown of the immunological system, ''but simply indicates a certain style of life.''

''The number of homosexually active males who share this life style,'' he said, ''may be much smaller than the number of homosexual males in the general population.''

Still another hypothesis under investigation is that the syndrome is related in some way to drugs, other environmental agents or some other common factors not yet detected.