At least 200 people are now known to have died in an outbreak of Japanese encephalitis in India's northern state of Uttar Pradesh.
Officials believe more could have been done to prevent the disease
Officials fear the death toll may be much higher as fatalities in rural areas often go unreported.
There is no specific cure for the mosquito-borne disease which has killed thousands in India since 1978.
But health experts complain that red-tape has prevented the development of a more effective vaccine.
In the past few weeks more than 500 people, mostly children, have been treated for the disease, which occurs regularly during India's monsoon.
Doctors say children between the age of six months to 15 years are worst-affected.
Most of the victims are very poor people from rural areas, says Dr TN Dhole, professor of microbiology in Lucknow's Sanjay Gandhi Post Graduate Medical Institute.
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.
Successive governments in Uttar Pradesh have failed to develop an effective programme of vaccination, the BBC's Ram Dutt Tripathi in Lucknow says.
Doctors report that hospitals are experiencing overcrowding
Dr Dhole says that the Indian Council of Medical Research (ICMR) has prevented field trials of a tissue-based vaccine that China has used to undertake mass immunisation.
He alleged that the ICMR blocked the trial in order to protect the monopoly of its own laboratory which manufactures a more expensive vaccine derived from the brains of mice.
No-one was available for comment at the ICMR when contacted by the BBC.
Japanese encephalitis, which causes high fever, vomiting and can leave patients comatose, usually hits the state at the end of August but this time it struck in July.
The disease has recurred annually in eastern regions of the state since 1978.