There have been at least 200 cases and possibly as many as 750 at one camp for homeless quake survivors in Azad Kashmir, amid fears it could be cholera, the World Health Organisation (WHO) and Unicef said.
"Rain would be disastrous," WHO emergency co-ordinator Rachel Lavy told AFP at the main camp on sports ground of the devastated university in the regional capital Muzaffarabad. About 3,000 people are living there.
Rain - the first for six days - started in quake-hit NWFP and parts of Azad Kashmir early on Thursday and was due to continue on Friday, while snow is expected at night, the met department said.
Winter weather poses the biggest threat to survivors of the October 8 quake, which killed 74,000 in NWFP and AJK.
UN Secretary General Kofi Annan has warned there could be a massive second wave of deaths.
The disaster left around three million people homeless. The numbers in the camps that have sprung up in almost every town and village are swelling as people coming down from the freezing Himalayan Mountains.
Aid workers said they were now focusing on preventing life-threatening diarrhoea at the camps in Muzaffarabad, by teaching people how to keep clean, digging new latrines and setting up an isolation tent for the sick.
Doctors in the ruined NWFP village of Batagram said they had dozens of pneumonia cases, while the United Nations said it was investigating possible diphtheria cases in a number of towns.
Claudia Hudspeth, Unicef's head of operations in Azad Kashmir estimated around 25 per cent - 750 people - at the university ground camp in Muzaffarabad had been affected by diarrhoea. She added there were more than 30 camps throughout Muzaffarabad, all of which could be affected.
The WHO's Lavy said there were at least 200 cases of acute watery diarrhoea at the camp in the last five days, including 55 reported on Tuesday and 77 the following day. There have been no deaths so far. 40 other cases were recorded last week in Chinari AJK.