It causes 180,000 deaths a year. Asthma is a chronic, inflammatory lung disease characterised by recurrent breathing problems. People with the disease suffer "attacks", or acute episodes, when the air passages in their lungs narrow and breathing becomes difficult. Attacks are caused by the airways over-reacting to certain environmental factors. They then become inflamed and clogged.
A definitive cause for the lung abnormality at the root of asthma has so far eluded doctors.
It is reported that hundreds of thousands of asthma sufferers have flocked to the Indian city of Hyderabad convinced that eating a medicine of live fish and special herbs will cure them.
The southern city's Goud family has been dispensing the medicine at this time of year for nearly 50 years. They say they received the formula from a Hindu saint. Sceptical scientists say the formula should be revealed for scrutiny but the family says it fears others will then exploit it.
The time of the treatment is fixed each year by astrologers and normally coincides with the arrival of the monsoon rains. Patients must swallow the live fish - a murrel or sardine - along with a ball of mystery yellowish paste, then go on a strict 45-day diet of 25 different foods, including lamb, rice, sugar, dried mango, spinach and clarified butter.
"If all these instructions are carefully followed we can guarantee a 100% cure for any patient no matter how bad their asthma," says family patriarch Harinath Goud.
Special trains and buses are laid on from across India to bring believers to Hyderabad and some patients travel from overseas.
Nearly 200 family members and volunteers administer the fish and about 5,000 police officers control the crowds.
The Gouds say the wriggling fish helps clear the accumulated phlegm in the throat.
However, doctors have urged the family to disclose the formula.
"If they refuse to do so, the government should withdraw its support for the event," said MV Ranga Redy of the Indian Medical Association.
Dr Sumanth Mantri, a chest specialist at Hyderabad's Apollo Hospital, said: "There is absolutely no clinical evidence to prove it works... this acts more as a psychological than a clinical cure. There can be some temporary relief."
The People's Awareness Forum, a group that tries to dissuade people from following rituals, says it will take the family to court if they continue to refuse disclosure.
But Bathini Harinath Goud, one of five brothers who make the yellow paste, said the remedy would lose its potency if commercialised.
"We will continue the distribution of the medicine until the last man gets it," he said.
Is there help?
The National Asthma Campaign has a helpline on 0345 010203, or you can write to them at Providence House, Providence Place, London N1 0NT for England or 21 Coates Crescent, Edinburgh EH3 7AF for the National Asthma Campaign in Scotland.