Other possibilities

While some medical experts believe the "patients" might suffer from some kind of psychological AIDS phobia, others have said the possibility of finding a new virus in the future cannot be ruled out.

Wu Zunyou, Director of AIDS Prevention Department at the CDC , said after the ministry's announcement that medical tests, especially for such complicated diseases, are not absolute.

In an earlier year-long blood test conducted by the Shanghai Institute of Virology, five people in the group were found to have the same unknown virus, Wu said.

"What I can say is that it is definitely not the virus that we know to cause AIDS. The virus' sequence from the five blood samples is similar, but we need more time to study exactly what it is," he said.

A sufferer with a pseudonym Lin Chang wrote on his online blog. "I have AIDS-like chronic fatigue syndrome. It has been clinically diagnosed. I believe that makes me living proof that AIDS is similar to this mysterious autoimmune disorder. And yet medical authorities appear unable or unwilling to acknowledge the possibility."

Three months later, his wife developed similar symptoms. The couple decided to send their 13-year-old son to his grandparents.

"I can pinpoint exactly when my undiagnosed illness spread from my body to another. I am the link in a chain of systemically undiagnosed, sexually-connected people. Whatever I am suffering, it strongly resembles the classic AIDS disease," he wrote.

He said many people with chronic fatigue syndrome do not like to talk about the immune abnormalities that they share with AIDS patients. Most patients would rather be told they have the loosely defined chronic fatigue syndrome than AIDS.

From March 31 to May 3, Zhong Nanshan led a team conducting a clinical investigation of 60 such patients.

The researchers did not detect HIV in all the patients, but Zhong did not entirely concur with the Ministry of Health's findings that the group was basically healthy apart from psychological issues.

"We admit that their conditions could be made worse due to psychological factors," he said. "However, we do not agree there was nothing wrong with the patients. We have found them carrying viruses that could be contracted via unprotected sex. A large majority of these patients' symptoms were triggered or became obvious right after sex."

But Zhong could not conclude they were simply suffering from sexually transmitted diseases. There was also no evidence to suggest a new, unknown virus was affecting the patients and further studies and tests would be needed before they could confirm a diagnosis.

Zhong urged people to avoid unprotected sex and to stop sharing dishes in order to prevent exposure to the potential virus.