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Outbreaks of Acute Encephalitis Syndrome and Japanese Encephalitis are common every year in India, especially during the monsoon season, and claim hundreds of lives.
But this year, major outbreaks - usually most prevalent in the northern states of Uttar Pradesh and Bihar - have spread to regions such as West Bengal and Assam further east and north - killing 568 people.
In West Bengal, where at least 111 people have died from both strains, a senior health official said authorities were taking emergency steps to contain the outbreak.
"We have sounded an alert in seven districts and cancelled the leave of all health department officials," West Bengal's Health Services Director B.R. Satpathy told the Thomson Reuters Foundation.
The health department has set up clinics across affected areas and is trying to prevent breeding of mosquitoes by fogging, especially around pig farms, where there is a high risk of contracting the virus.
Encephalitis is an inflammation of the brain, caused by any one of a number of viruses, says the World Health Organisation. Symptoms include high fever, vomiting and, in severe cases, seizures, paralysis and coma. Infants and elderly people are particularly vulnerable.
It is most often caused by eating or drinking contaminated food or water, from mosquito or other insect bites, or through breathing in respiratory droplets from an infected person.
Outbreaks of the virus tend to occur in poor, flood-hit areas, where monsoons have left pools of stagnant water, allowing mosquitoes to breed and infect villagers.
Floods also lead to the contamination of clean water sources such as wells, leaving many people with no option but to use the same dirty water for both drinking and sanitation.
Health Minister Harsh Vardhan said last month that he was distressed at the "runaway conquest of encephalitis" and ordered the vaccination of all children in vulnerable states and the provision of dedicated hospital beds.
In 2012, the government launched a national programme to prevent and control the virus, including expanded vaccinations, strengthened surveillance and improved access to safe drinking water and sanitation.
There were 1273 deaths due to encephalitis in 2013 compared to 440 deaths from malaria and 193 from dengue, according to government statistics.