The Chinese authorities have been accused of covering up respiratory illnesses like Sars in the past.
This time doctors are blaming a breakdown in trust between the medical profession and patients, who fear they are being lied to when their diagnostic tests come back negative.
One man convinced he has the condition insisted on meeting in an empty motel room. He tries to avoid public places to reduce the chances of transmission.
He wears a face mask - he suspects his virus is spread by close contact, through sweat or saliva. He thinks he caught it after he had sex with a female prostitute.
But he is not HIV positive - seven HIV tests have come back negative.
"I've been to many hospitals, I've had many tests. None of these has proved I'm ill," he explains.
"They've examined my organs, tested me for sexual diseases. I'm unwell, but the doctors can't explain why."
There are dozens of Chinese internet chat rooms filled with people who believe they have the same mystery illness.
"I joined the chat room because I was sure I had been infected with this virus," said another patient, who refused to meet face-to-face because, he said, he did not want to pass it to us.
"Twenty-four hours later I had a strong desire to vomit. I had headaches, I was dizzy, I could feel my internal organs were swelling up. I was in intense pain. This lasted months."
He thought he was HIV positive but was tested several times and there was no sign of HIV antibodies.
The man is unhappy with the response from the medical establishment in China and has tried to bring his illness to the attention of the World Health Organization (WHO) and researchers overseas, but with little success.
"Most of the doctors didn't have the patience to listen to my story," he complains, adding that he is sure the virus is spreading throughout the country.
Both men are certain they are ill, but at the moment doctors do not think they are dealing with an unknown virus.
Dr Cai Weiping
Scientists at the Pasteur Institute in Shanghai started getting letters from the patients in August.
In early December, they began a study of five patients. So far they have ruled out HIV. The work is still continuing.
Last month, China's Centre for Disease Control tested a larger group of 60 patients. They too ruled out any connection with HIV.
Dr Cai Weiping is a senior Chinese HIV researcher based at the People's Number 8 hospital in the southern province of Guangdong.
He is concerned that growing numbers of patients with what he describes as "HIV phobia" are using up scarce resources.
"They come to have tests again and again, wasting money.
"A real HIV sufferer may take 15 minutes to deal with. A patient with the phobia can take at least an hour, or as much as half a day of arguing before they go away."
Some of the patients claim they have infected family members, friends or colleagues. Dr Cai is doubtful.
"What their relatives tell us about their own symptoms doesn't match what we have heard from the patients."
He believes the problem is psychological rather than physical.
"They think we are concealing an epidemic," he explains.
"In the past we were secretive about the spread of diseases. People didn't believe the numbers of infections we announced.
"Today that's impossible because China is now making much more effort to find patients who have HIV or other diseases."
Although incidences of "HIV phobia" have been reported in other countries, the doctor believes conditions unique to China have produced a larger number of cases here.
They have left many patients suspicious of the motivations of the medical profession.
"Patients think doctors just see them as machines to make money out of, instead of being driven by a desire to cure them or to save life," says Dr Cai.
The internet has allowed large numbers of people who are frightened but have little expertise to share their fears and in the process heighten them.
But even if the doctor is right and the young man in the motel room is suffering simply from delusion, it is severe enough to leave him trapped behind his mask.
"I feel that I will die soon," he says.
"I haven't been home for a month because I don't want to infect my family. My doctors don't understand me. They say it's caused by fear, but my symptoms are real."
He is so scared he might spread what is wrong with him to others, he has started to withdraw from society.
Physical or mental, the effect of this condition is devastating for him.