JOHI, Pakistan (AFP) — His head haloed by a whirlwind of mosquitoes, Aamir Hussain stands on the roof of his home in southern Pakistan surveying the fetid floodwaters all around.

Four months after the start of record monsoon rains linked to climate change, the standing water has curdled into a pestilent soup breeding malaria, cholera and dengue.

The United Nations has warned of a "second wave" of catastrophe, with the risk that deaths from water-borne disease and malnutrition will outstrip the 1,700 drowned and electrocuted in the initial cascade.

As dusk arrives in Hussain's submerged village in Dadu district of Sindh province, so do the bugs, and the gamble that they will infect his wife and two children.

"The mosquitoes bite a lot and we fall sick," the 25-year-old said.

His brother, who shares this home, has already ventured off the roof to treat his sick children at hospital with borrowed cash.

"Some of our nets are torn now so we are worried," Hussain, whose infant son has fallen ill, said.

Sindh has been worst-hit by the catastrophic flooding which put a third of Pakistan underwater, displaced eight million, destroyed or damaged two million homes, crippled 1,500 hospitals and clinics and caused an estimated $28 billion in damages.