Nearly 130 people are known to have died from Japanese encephalitis this year in eastern parts of the Indian state of Uttar Pradesh, officials say.
A senior health official told BBC that 640 people were admitted in hospitals for encephalitis, known locally as brain fever. Out of these, 129 died.
Doctors say that the high death ratio is "quite high" and "alarming".
Japanese encephalitis is caused by a virus. It is passed to humans by the bite of infected mosquitoes.
It cannot be transmitted by other humans and is usually a mild illness - in many cases there are no symptoms.
But in a small number of cases - normally about 1 in 200 infected people - the illness is much more serious.
A senior doctor in Gorakhpur Medical college - near the border with Nepal - said that the majority of victims in the latest outbreak were children.
They say there is not enough nursing and support staff to attend to them as new patients arrive every day.
Flood waters from Nepal combined with long standing water logging in Uttar Pradesh is has been blamed for extensive breeding of mosquitoes.
More serious infections may start with fever, tiredness, headache, vomiting, and sometimes confusion and agitation leading to encephalitis (inflammation of the brain).
It can cause permanent brain damage in some cases.
Eastern Uttar Pradesh is a poor and backward area without adequate health facilities and encephalitis has killed a large number of people there over the last 30 years.
One of the worst outbreaks happened in 2005 when hundreds of people were killed.
Deforestation and increasing sugar cane farming have added to the problem.