The once rare form of pneumonia now commonly found in AIDS patients has been discovered in a group of five elderly hospital patients, none of whom had AIDS or any other medical condition that has been linked to the lung infection.

All five developed their pneumonia within a three-month period, and three of them died, according to a study published last week in the New England Journal of Medicine.

Doctors at the New York Hospital-Cornell Medical Center say "it is possible that these patients were infected with a more virulent strain of pneumocystis carinii," the organism responsible for AIDS-related pneumonia.

Until now, virtually every case of pneumocystis pneumonia was found in people whose immune systems have been crippled, either through AIDS, leukemia, malnutrition or various drug treatments.

Further tests will be needed, they said, to discover if the organism has found a way to transform itself into a more infectious form.

"We should soon know whether this cluster of patients is unique or is the first indication that the epidemiologic nature of {Pneumocystis} carinii infection is changing again," said the team, led by Jonathan Jacobs, director of the hospital's AIDS center.

The cluster caught the attention of doctors at the hospital because pneumocystis pneumonia is so rare in the general population and "these five patients were very unlikely candidates for this type of pneumonia," Jacobs said. Before the mid-1960s, only 100 cases had been reported in the United States.

Jacobs said the researchers stumbled onto the cases because the hospital, which treats many AIDS patients, now routinely looks for the pneumocystis organism in every person in whom the cause of pneumonia can't be easily found and where standard antibiotic treatments don't seem to be working.

In an editorial, Peter Walzer of the University of Cincinnati College of Medicine said doctors should now look for pneumocystis in any elderly person suffering from pneumonia.