By John Sudworth BBC News, Dhaka

Sickness is now the overriding concern in many parts

When I spoke to her from the deck of the small motor launch we had hired for a trip up the Jamuna River, Panu Begum was sitting on the roof her house.

She had been living on the small strip of corrugated tin for five days. The water below was too deep to stand in.

"It's entered everyone's home," she told me. "There's nowhere to take refuge. I am cooking, eating and sleeping up here."

Next door, her neighbours, a family with four children, huddled together under what little shade they had from the heat of the sun.

This whole family had also been on their roof for the best part of a week.

Their goat was the only valuable possession to have been saved from the flood.

No stranger

Dozens of families in this island community in the district of Manikganj are in the same predicament.

Whole communities are now exposed to flood water and are getting sick Dr Alejandro Cravioto

Some have been given a few kilograms of rice, but they complain it is not enough.

There are boats of course. But many prefer to stay on top of their homes rather than brave life in an emergency shelter, or even worse, in the open.

Every year about one fifth of Bangladesh disappears beneath the monsoon rain waters. The country is no stranger to floods.

This year more than 40% of the land has been affected.

Yet even now the situation is not as bad as it was in 1998, the year of the last so called "mega-flood".

Stagnant water

But in some areas the crisis is acute and the floods are causing real hardship.

Aid agencies say supplies are reaching those in need. The UN's World Food Programme has now distributed some 130 tonnes of aid to the north of the country, and has begun a second phase around the more recently flooded central districts.

Panu Begum: "I am cooking, eating and sleeping up here"

But the real concern now, as across all the flood-hit parts of South Asia, is the danger of water-borne disease.

You have to wade through brackish, stagnant flood water to reach Shajeda Akhter's home on the outskirts of Dhaka.

She was cooking a meal when we arrived, boiling vegetables in the same dirty water scooped from the flood below.

The family knows it is taking a risk with the health of the two young children. But there is no other choice.

The tube well is flooded. There simply is no clean water available.

"If my children get sick I will call a doctor," Shajeda tells me.

Mercy dash

The International Centre of Diarrhoeal Disease Research in Dhaka normally takes around 150 patients a day.

In the aftermath of the floods, more than 600 new cases are now coming through the doors every 24 hours. Many are seriously ill.

During my visit one woman was rushed in just moments from death.

There are 600 new diarrhoea cases a day at this Dhaka centre

Doctors thought she might not survive, but their quick intervention and the rapid re-hydrating saline solution fed into her arm kept her alive.

The hospital has had to build a large tent, with bamboo poles and canvass sheeting, to cope with the influx. No-one will be turned away.

"These people are coming from the flooded areas of Dhaka city," Dr Alejandro Cravioto tells me. He believes numbers will still rise further.

"At first it was mostly adult men who were probably drinking contaminated water at work," he says.

"But in the last few days we've had an increase in the number of children, meaning that whole communities are now exposed to flood water and are getting sick."

The forecasts suggest the flood waters will be gone within 10 days, but the health risks will remain for some time longer.

In some cases children get better and leave the hospital only to return to homes where there is still no clean drinking water.