An outbreak of Japanese encephalitis in the Indian state of Uttar Pradesh has claimed the lives of nearly 700 children, according to official figures.
A mother waits anxiously with her child
Aid agencies say the real toll could be closer to 1,500.
They allege local health workers could be under reporting the number of deaths in their areas for fear of punishment for not halting the spread of the disease.
When the BBC's Ram Dutt Tripathi visited Baba Raghavdas Hospital in Gorakhpur he found two or three children to every bed in one ward.
Many were unconscious and their anxious parents were applying cold wraps to their forehead while nurses adjusted saline drips.
"This is unprecedented," says Professor KP Kushwaha at the hospital. "I have seen 15 to 20 children dying every day. I am pained because I am seeing them dying with my own eyes and sometimes I feel helpless."
The ward has just 48 beds. Around 50 new patients have been coming in every day and the hospital is overwhelmed, our correspondent says.
The situation is repeated elsewhere.
Professor Kushwaha says there are 100 nursing homes in Gorakhpur and all are full to capacity.
And there is an acute shortage of doctors and nurses.
The outbreak began earlier in the summer when the monsoon rains started.
The first cases of Japanese encephalitis were reported in India in 1978.
Since then around 8,000 people have died from the disease in Uttar Pradesh and the neighbouring state of Bihar.
The virus is transmitted by mosquitoes.
They transfer it from infected animals - usually pigs and wading birds - to humans.
The disease causes high fever and aching. Eventually victims fall into a coma and nearly a third will die.
Health officials say pig farms must be moved away from villages to prevent outbreaks.
But Chief Minister of Uttar Pradesh Mulayam Singh Yadav relies on the votes of the rural poor, and critics say he is reluctant to take any step that could endanger their livelihoods.
High child mortality
This outbreak of Japanese encephalitis, which is far worse than in previous years, has highlighted India's inability to deal with preventable diseases despite its increasing wealth.
On Wednesday the United Nations released its annual development report which showed that for every 1,000 Indian children born, 63 die before the age of five.
Many catch the disease from mosquitoes via infected pigs
And Uttar Pradesh is one of four Indian states with the worst figures.
The child mortality rate is far greater than in neighbouring Bangladesh, which is a much poorer country.
One problem is a lack of vaccines.
This year just 200,000 children out of seven million have been immunised in the areas of Uttar Pradesh most vulnerable to Japanese encephalitis.
The Indian government is trying to act.
Health Minister Anbumani Ramadoss has visited Uttar Pradesh to launch a rural health programme that will include mass immunisation.
"There should not be even one case of Japanese encephalitis in India," he said.
And he criticised India's health officials for not stopping outbreaks, saying it was the government's responsibility to eradicate preventable diseases.
But India's only laboratory, in the Himalayan town of Kasauli, produces just half a million vaccine doses a year.
That is not nearly enough for the country's huge population.
Now India is trying to import more effective vaccines in larger quantities from China and South Korea.