Shahdadkot, Pakistan (CNN) -- For almost a million Pakistanis, the misery of epic flooding covering one-fifth of the country has now taken the form of communicable illnesses.
Cases of acute diarrhea have topped 204,000, the World Health Organization announced Sunday. The number of skin diseases -- such as scabies -- has topped 263,300.
More than 204,600 Pakistanis have reported acute respiratory infections as filthy waters surround homeless flood victims, WHO said.
Thousands have cases of suspected malaria.
"Strong water and sanitation interventions, such as providing clean drinking water supply and addressing environmental hazards, are urgently needed to prevent outbreaks of waterborne diseases in" Charsadda, Nowshera and Peshawar, WHO said.
Prime Minister Yousaf Raza Gilani has called a meeting for Tuesday of senior Pakistani health officials, local officials, U.N. agencies and other international group to look into the emerging health crisis, state-run TV reported.
Meanwhile, the half-million residents of Shahdadkot frantically tried to flee their homes over the weekend as a wall of water threatened to burst mud berms and drown the entire city in Pakistan's Sindh province.
Three weeks into the worst natural disaster in Pakistan's history, people were still desperate to escape as a second wave of monsoon floodwaters surged southward. More than 1,500 people have died and 20 million lives have been disrupted.
Residents climbed onto heaps of belongings piled high in the beds of rickety trucks, packed buses, auto-rickshaws and carts to get out of town before the water came. Many did not know where they were going -- just that they had to reach dryer ground.
But there weren't enough vehicles for a mass evacuation.
Sunat Magsi and her 100-strong extended family lost their nine mud huts to the raging torrents. They sought shelter in an abandoned house but even there, the water was creeping higher. They only had one donkey and one cart left.
"We have so many children here," Magsi said, weeping. "We don't know how we're going to get out. We need help."
Pakistan is dotted with villages, towns and cities submerged like Shahdadkot. Floodwaters are expected to recede in the next few days as the last surges in the Indus flow into the Arabian Sea.
But the suffering is sure to continue. Health officials fear that the human toll will get a lot worse as people are forced to wade through unsanitary water while clean drinking water is scarce.
More than 200 health facilities have been damaged or destroyed, according to the World Health Organization, greatly reducing the available health care for millions of survivors in filthy conditions. At least 4 million people are homeless.
"The depth of suffering is incalculable as risks escalate of diarrhea, acute respiratory infection, malaria and other communicable diseases," said Dr. Guido Sabatinelli of the World Health Organization. "It is crucial that all humanitarian health providers, local and national, coordinate their relief efforts closely to save lives, reduce suffering and deliver the most effective response."
WHO said waterborne, airborne and vector-borne diseases, including acute watery diarrhea, measles, malaria and acute respiratory infections, are threats due to overcrowding, lack of hygiene and breakage in waterlines.
The focus is now turning to heading off more disease, particularly among young children, WHO said.
More than 100,000 children have been given polio shots in Charsadda and Peshawar regions in the first three days of a new vaccination program.
United Nations officials have appealed for $460 million over the next three months to help the roughly 20 million people in need of shelter, food and emergency care.
Imram Khan, a political activist and former Pakistani cricket player, said the real damage from the flooding will be witnesses once the water subsides.
"I don't think the international community fully comprehends the extent of the disaster, because ... the real problem is going to come when the water recedes," Khan told CNN's "Fareed Zakaria GPS" on Sunday. "The biggest fear is that about 20 million people will be completely impoverished. Now, most of these are subsistence farmers, so they have lost their crops. They have lost their animals."
The area affected by the flooding is also Pakistan's greatest area cotton crops, he said.
The desperation at camps for evacuees is such that people fight over aid trucks that come in, Khan said.
If the situation is not stabilized soon, the situation could become dire, he warned.
"The consequences in three months of this -- if this is not addressed by us -- the country is going to go into chaos," Khan said. "We could implode. We could have starving people. And you're talking, again, about 20 million people in dire straits. Where are they going to go with no food, no homes, no money, no crops, no animals?"
Meanwhile, International Monetary Fund officials said they will meet with members of the Pakistani government in Washington next week to discuss the economic impact of the massive floods that have ravaged the country.
"The floods, which have hit Pakistan in recent weeks and brought suffering to millions of people, will also pose a massive economic challenge to the people and government of Pakistan," said Masood Ahmed, director of IMF's Middle East and Central Asia Department.
Ahmed says the meeting will be an opportunity to evaluate the economic impact of the floods, assess what Pakistan's government is doing to respond to it and "discuss ways in which the IMF can assist Pakistan at this difficult juncture."
CNN's Kyung Lah contributed to this report.