Millions of people affected by flooding in South Asia face a health crisis unless relief work is urgently stepped up, the United Nations has warned.
Water sources in affected areas are either contaminated or submerged
The World Health Organization (WHO) and Unicef said stagnant waters were "a lethal breeding ground" for diseases such as malaria and dengue fever.
Food, drinking water and medicines are being distributed but the scale of the disaster has dwarfed relief efforts.
About 28m people have been affected by floods in India, Bangladesh and Nepal.
"Entire villages are days away from a health crisis if people are not reached," said Unicef's health chief in India, Marzio Babille.
THE ASIAN MONSOON
Monsoon winds blow north-easterly for one half of the year, and from the south-west for the other half
South-westerly winds bring the heavy rains from June to Sept
Winds arrive in southern India six weeks before the north west
Annual rainfall varies considerably
"Stagnant waters left by the floods are a lethal breeding ground for diarrhoeal and waterborne diseases at potential epidemic level."
Children, who make up 40% of South Asia's population, were particularly vulnerable, he said.
India is the worst-affected country, with some 20 million people in the states of Assam, Bihar and Uttar Pradesh hit by the floods, according to the UN.
Some eight million people are affected in Bangladesh and another 300,000 people in southern Nepal.
Although water levels are receding in Nepal and Bangladesh, millions of people are still marooned on high ground.
Dhaka resident Ishak Shohel, 27, told the BBC News website that rising floodwaters had forced her family of five to flee their suburban home.
They are now staying with relatives on higher ground.
"Until recently no relief supplies have been delivered to the affected people of Dhaka and its suburbs," she said.
FLOOD HEALTH RISKS
Waterborne diseases: cholera, leptospirosis
Insect-borne diseases: malaria, dengue fever
Dehydration, diarrhoea, gastroenteritis, skin infections
Children are particularly susceptible
The international aid agency Oxfam has launched a $2m appeal to help about 200,000 people across South Asia with essential food, clean water and shelter.
Oxfam spokesman Sam Barratt, who was in Orissa state last week, told the BBC:
"I saw for myself these massive scenes of devastation where the water has just smashed through banks which are 30 to 40 foot high (10-14m) and flooded villages, polluted wells, taken everything that people have got."
Most water sources in affected areas are said to be either contaminated or submerged.
Unicef's Marzio Babille told the BBC that an acute shortage of helicopters meant that the Indian authorities were finding it difficult to reach victims, who have spent more than a week in the open.
There has been a brief pause in the rain but the flood season is likely to continue for several weeks.
Indian Home Minister Shivraj Patil and Congress Party leader Sonia Gandhi visited the worst-hit region, in Bihar state on Tuesday, as anger rose over aid efforts.
About seven million people are affected in the state, and there have been reports of fights over limited supplies and of local officials stealing them.
The UN said that in Bihar alone, it needed many more helicopters for relief operations and at least one million more oral rehydration sachets to alleviate diarrhoea.