The outbreak occurred in Siliguri in the Himalayan foothills and killed about 70% of the people infected. Over 100 people are thought to have contracted the illness.

Nihar Ranjan Haldar, a neurologist in private practice in Siliguri, saw the first seven patients. “Most of them had coma, and fever. They went into fits, and started dying. In the beginning they all died, but it slowed down and at the end of five weeks, we saw no more deaths.”

Since the deaths stopped, Dr Haldar has been working with other investigators trying to find the cause of the disease. He presented their findings on Tuesday at the World Congress of Neurology in London, UK – and so far, they know very little.

Hide and seek

They presume the disease is caused by a virus as they found no sign of parasites or bacteria, and as the symptoms to some degree resemble those of Japanese bee encephalitis. But if it is a virus, it has remained resolutely hidden.

“The local investigations we have done with cerebro-spinal fluid, with blood – everything was negative,” Dr Haldar told żěè¶ĚĘÓƵ. “So we enlisted the help of several national research institutes. So far they have not found any virus which has already been reported. It could be a new virus.”

“It’s certainly serious – the mortality rate of 70% puts it level with Ebola or Marburg,” says Professor Thiravat Hemachudha, a neurologist from Chulalongkorn University in Bangkok, Thailand who chaired the conference session. “It’s highly contagious, and it must be something unfamiliar to us.”

The World Health Organisation and the US Centres for Disease Control have both been involved in searching for answers to this riddle, but Dr Haldar called for more intensive efforts.

The identity of the virus, he said, is only part of the mystery. “We know very little about how it behaves. We tried to track it – but we could not find the vector or the mode of transmission. Also we have no idea about the origin. The people who survived must have antibodies to whatever it is, and we need to study them urgently.”

These parasites infect around 1.5 billion people worldwide and, according to research from Nottingham University, UK and Jimma Hospital, Ethiopia they protect their hosts against asthma.

Asthma affects anywhere between six and 20 percent of people in developed nations, and is predominantly an urban disease. But developing countries are catching up, as more and more people migrate to cities.

In the study area in south-western Ethiopia, around five percent of people have asthma in the city of Jimma, but only half a percent in the surrounding rural districts.

Using a database from a previous study, the researchers identified 206 adults with asthma and compared them with 398 randomly selected controls. Some came from the city, others from outlying villages and the study controlled for age, gender and location.

The team found that infestation with two kinds of intestinal parasite, hookworm and the large roundworm Ascaris, both decrease a person’s risk of asthma. A heavy hookworm burden lowers risk by around two-thirds. Another common parasite type, Tricuris or whipworm, offers no protection.

Previous studies have connected intestinal parasites with immune response related to asthma. But Sarah Scrivener from Nottingham University, who led the study with Jimma Hospital’s Haile Yemaneberhan, told żěè¶ĚĘÓƵ that the mechanism behind this protection remains to be found.

“Ascaris and hookworm both migrate through the lungs as part of their life cycle,” she said. “And they may block the allergy response which causes asthma. One idea is that some hookworms can digest eotaxin, a substance that primes immune cells in the lung for action. And some kinds of hookworm secrete proteins which can damp down the immune response.”

“It’s an important finding and it’s consistent with what our group has seen in China,” says Scott Weiss, Director of Respiratory, Environmental and Genetic Epidemiology at Brigham and Women’s Hospital in Boston. “The importance for public health is huge – all over the developing world people are trying to eradicate worms.”

Recently the Gates Foundation donated $18 million to the Sabin Vaccine Foundation to search for a hookworm vaccine – a vaccine which Sarah Scrivener believes would be of major benefit to developing countries.

“But if they do get a vaccine it’s going to be important to find out what effect it would have on asthma,” she says. “Depending on how it works, it could either raise the risk by eliminating the worms, or maybe reduce the risk by mimicking them. But we need to know.”

The research was presented on Monday at the American Thoracic Society’s annual meeting in San Francisco.