Around two million people living in the southern region of Pakistan suffer from various waterbone diseases after the monsoon rains inundated the area, the country's disaster authority said.
More than 350 people have been killed and over eight million people have been affected this year by floods that officials say are worse in parts of Sindh province than last year, when the country saw its worst ever disaster.
Malaria, diarrhoea, skin disease and snake bites were among the health problems facing two million people across 23 Sindh districts, said Irshad Bhatti, spokesman of National Disaster Management Authority (NDMA).
"In some areas, diseases also spread out because of dead animals but there is no major break-out of any epidemic," Bhatti added, calling for the donation of mosquito nets and medicines to help the aid effort.
The World Health Organization (WHO) has said there is a desperate shortage of clean drinking water in the south which has also triggered outbreaks of acute diarrhoea and other waterborne diseases.
The UN's children agency has pledged to distribute 200,000 litres (50,000 gallons) of water to 40,000 people daily and deploy 40 more water tankers soon, to avert further disease.
Parts of Sindh are under several feet of water and Pakistan's meteorological department says the worst-affected districts of Badin, Mirpurkhas and Thar seeing have seen eight times the usual levels of rainwater.
The government is trying to fend off criticism of an inadequate response to the floods one year after it was castigated for failing to respond quickly enough to its worst ever floods that affected a total of 21 million people.
For months, aid groups had warned the government to invest in adequate prevention measures to mitigate against seasonal rains, avoid a repeat of last year's $10 billion losses, and protect those left vulnerable two years running.
Calling on the international community to step in to help once more, the UN and Islamabad on Sunday jointly issued an emergency funding appeal for $357 million for emergency rescue and relief efforts.
So far only the Japanese government has declared a pledge, of 10 million dollars, the UN said.
"We appeal to international community and other donor countries to come forward and help the the people devastated by the floods in Pakistan," said UN spokeswoman Ishrat Rizvi.
Zafar Qadir, chairman of the NDMA, said that more than 550,000 people had been made homeless in the south of the country and called for urgent donations of tents.
"Over five million people are in camps and we need an immediate supply of tents to accommodate all of them. We are facing a severe shortage of tents," he told a news conference.
He said that Japan has sent 3,500 tents so far.