Hundreds of earthquake survivors living in camps in Pakistani-administered Kashmir have acute diarrhoea, World Health Organization officials say.
Conditions in some of the camps are very poor
Doctors are investigating whether the outbreak has been caused by cholera.
The UN has said 350,000 people urgently need shelter before the onset of winter and medical aid is still to reach many others living in remote areas.
Pakistan's official death toll is 73,000 although donors and aid agencies have placed the figure much higher.
"In one camp we visited yesterday there were 55 cases of diarrhoea and there are so many spontaneous camps that we believe there are hundreds of others," WHO worker Rachel Levy told the AFP news agency in Muzaffarabad.
Doctors in the city are trying to contain the outbreak before it spreads further.
They are treating the most serious cases with intravenous drips and making efforts to improve water supplies and sanitation in the tent camps which have sprung up around the city.
An estimated three million people are homeless in the earthquake. UN officials have warned that the death toll may rise further as winter approaches.
Confusion over the official figures emerged on Tuesday after a finance ministry adviser, Iqbal Ahmed Khan, put the death toll at 87,000.
The UN says winter is now the greatest threat to survivors
He said his information came from officials from the World Bank and Asian Development Bank.
The banks, working with local government and aid agencies, said they found previously unlisted casualties in areas which had been cut off by landslides, Mr Khan said.
However, a spokesman for Pakistan's Federal Relief Commission told the BBC News website that the official toll was still just over 73,276.
In addition, India says nearly 1,400 people were killed in Indian-administered Kashmir.
On Wednesday, a second crossing opened on the Line of Control (LoC) that divides disputed Kashmir between India and Pakistan.
But Kashmiri residents are still unable to cross the LoC to help relatives and loved ones on the other side.
India and Pakistan have struck a deal to open five points along the heavily militarised LoC to help earthquake victims, but procedural difficulties are slowing things down.