At least 200 children have died in an outbreak of Japanese encephalitis in northern India, health officials say.
So far, 900 affected children have been admitted to hospitals in Uttar Pradesh state. Some patients have come from neighbouring Bihar state and Nepal.
There is no specific cure for the mosquito-borne disease which has killed thousands in India since 1978.
Health experts complain that red tape has prevented development of an effective vaccination programme.
The disease occurs regularly during India's monsoon.
Doctors say children between the age of six months to 15 years are worst affected and most of the victims are poor people from rural areas.
"The attack of the encephalitis virus is extremely ferocious this year," says Dr Rashmi Kumar, an expert on Japanese encephalitis at Lucknow Medical College hospital.
"Children are developing a serious condition within a day or two of getting infected," she says.
Health officials in the state capital, Lucknow, say cases of acute encephalitis are being reported mostly from 14 districts of eastern Uttar Pradesh in the foothills of the Himalayas.
The low-lying areas are prone to annual floods, and severe water-logging and a lack of sanitation provide a breeding ground for mosquitoes.
Doctors say Gorakhpur town is the epicentre of the disease.
Last year, the government said it would spend 60 million rupees ($1.24m) to upgrade facilities at Gorakhpur Medical College hospital.
But, doctors say, the hospital does not have adequate numbers of medical staff to deal with the large numbers of patients.
Doctors say the children who survive will have to face lifelong problems as the disease has a crippling effect.
While there is no specific cure for the disease after it has been contracted, three vaccines are in use worldwide that have reportedly been successful in preventing the disease.
But India has so far failed to develop an effective vaccination programme.
After the disease killed 1,500 children in 2005, a public outcry forced the government to import vaccines from China and a mass vaccination project was started.
However, doctors say the vaccine coverage has not been satisfactory this year, with many parents of affected children saying no vaccination was done in their areas.
Japanese encephalitis, which causes high fever, vomiting and can leave patients comatose, usually hits Uttar Pradesh state in July-August.
The disease has recurred annually in eastern regions of the state since 1978.