Three-year-old Sammy Kushnick had the best that money could buy and more: adoring parents who had him and his twin, Sara, in later life; a sprawling home in the Hollywood Hills and the latest in playthings; a carefully selected nursery school at a Beverly Hills temple and, when he became ill, the finest of doctors and hospitals.

When he died last October, it was of a disease not mentioned in the child-rearing manuals on Jerry and Helen Kushnick's bookshelves, a disease that didn't occur to the pediatricians.

Sammy died of AIDS--acquired immune deficiency syndrome--a disorder that leaves the body prey to infection and is almost always fatal. Four of five children in Los Angeles County who have contracted it are dead.

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In part, Sammy's story is a medical mystery--about why doctors had difficulty recognizing the disease and about exactly how he contracted it. But it also is the story about the effects of such a disease on the surviving twin, on the parents and on the community.

The public thinks of AIDS as a disease of homosexuals, Haitians and intravenous drug users; medical authorities say it is spreading to the general population. It still is rare among children, one of the few pediatric AIDS specialists in the nation said.

Tainted Blood Suspected

Sammy is thought by county health officials to have gotten in from tainted blood he received as a newborn at Cedar-Sinai Medical Center, blood transfusions that may have saved the sickly infant at the time but that may have given him a disease that took three years to show up.

From a medical point of view, Sammy's illness may have been unavoidable because there is no test that can detect AIDS in blood donors.

Once over the initial grief, the family got angry.

By their account, here's what happened:

· Several pediatricians did not recognize that something was very wrong, dismissing Sammy's fevers, colds, diarrhea and earaches as common childhood maladies.

· When the Kushnicks tried to get assurances that the blood given to Sammy and to Sara as well had been traced and tested, they ran into a brick wall. Citing obligations of confidentiality, they said, the hospital as well as the county Department of Health Services, the Red Cross and the Centers for Disease Control declined to provide them with information.

· The nursery school that the twins attended treated the family "like lepers," the Kushnicks said. They said Sara was kept out for fear she might "contaminated" the other children, despite immunological tests on the family and a letter from the hospital attesting to Sara's excellent health.

"The community just went berserk," Dr. Shirley Fannin associate deputy director of Los Angeles County's communicable diseases control program, said of Sara's treatment at the nursery school.

"The more I think about the way we were stone-walled, first as Sammy lay dying and then afterwards as well, the angrier I get," said Jerry Kushnick, an attorney who runs a theatrical management company in West Hollywood with his wife.

Helen and Jerry Kushnick now 38 and 52 respectively, had been married less than two years when Sammy and Sara were born two months early, a little more than three pounds each. The preemies spent six weeks in neonatal intensive care unit. Sara flourished; Sammy, whose lungs were somewhat immature, developed a serious bacterial infection. Both children received blood transfusions--replacements, actually, of tiny amounts of blood.

Finally, Sammy was allowed to go home and the Kushnick family began the hectic adventure of rearing twins.

For two years they thrived, their parents recalled.

Then Sammy started falling behind Sara in height and weight. He seemed constantly to have a cold, a runny nose, an ear infection, a high fever, a distended stomach, diarrhea, thrush, viral infections, coughs.