The naturally occurring poison is being pumped out of the ground in wells, many of them paid for by the international community.

More than 40,000 villages in the south and west of the country are affected. Their water comes from tube wells sunk into the ground to depths of up to 200 feet.

The source is natural underground reservoirs, called aquifers. But in many cases arsenic in the sediment contaminates the water.

The BBC's South Asia correspondent, Mike Wooldridge, discovered the anxiety behind the early morning rituals of villagers in the district of Faridpur. He says this is what Bangladesh, with its history of natural calamities, needs least of all.

In one village, medical workers find 14 of the 15 wells are well above the arsenic limit

The only way to avoid exposure to poisoning, people have been told, is to stop drinking water from these wells. Uncontaminated wells, though, are few and far between.

Beyond a certain point the disease can no longer be checked and increasing weakness sets in. For most villagers, there is resignation, too. "Wherever I dig, the water is contaminated," said one.

In a nearby village the tube wells are tested by a team from a Bangladeshi charitable medical network. Out of 15 examined, 14 are well above the arsenic limit and more cases are surfacing.

'Why should I work when I'm going to die,' asks this villager

One young father has exhausted his money trying to treat what he thought was a skin infection. Doctors say cases like these are usually incurable and his outlook is poor.

Dr Sharif Tushar, from Dhaka Community Hospital, told BBC News: "We visit different villages every day and we find more contaminated wells and new patients. Some can be cured - some cannot. The situation is getting worse."

Red paint is a tell-tale sign that this well is contaminated

Arsenic contamination is found in other parts of the world but only in Bangladesh are so many people at risk. It looks like there has been a terrible price to pay for efforts to extract more and supposedly safe water from beneath this impoverished country.

Life is notoriously tough for many in Bangladesh and tube wells have been vital in attempts to transform the country from basket case to bread basket.

The charity UNICEF, which has helped sink more than a million wells, acknowledges that despite warning signs the arsenic crisis crept up on the international community.

Black marks on hands and feet are symptoms of arsenic poisoning

Deepak Bajracharia, from the charity, said: "The gravity of the problem was not recognised as well as it should, or could, have been.

"It's difficult to pinpoint where the blame lies. I don't think it was done maliciously on anybody's part - it was done with the best intentions of better health for people."

As another household sees its well declared unsafe, there are nagging questions about whether this nation might have over-exploited its ground water and whether the crisis could have been anticipated.