With its sewage system badly damaged, carcasses rotting in the rubble and thousands of people sleeping rough, experts say Nepal faces a race against time to ensure a devastating earthquake does not trigger a public health disaster.

More than 5,500 people were killed in Saturday's quake and relief officials will be desperate to avoid a second tragedy akin to a cholera outbreak traced back to UN Nepalese peacekeepers that devastated Haiti after a quake in 2010.

"When you've got an environment where hygiene is poor and people are drinking water from dubious sources there's always going to be a risk of water-borne diseases, diarrhoea and respiratory diseases," Patrick Fuller, Asia-Pacific spokesman for the International Red Cross, told AFP.

In Nepal's ruined capital Kathmandu, thousands have spent five nights so far camped out in the cold, crammed into tents without access to safe, clean drinking water and flushing toilets, forced to defecate in the open.

Fear of disease is sweeping through the camps, with the homeless donning surgical masks and even Nepalese Prime Minister Sushil Koirala wearing one as he toured tents trying to reassure terrified families.

"Sanitation and water facilities at these camps is a real concern," Dr Babu Ram Marasini of Nepal's Epidemiology and Disease Control body told AFP.

"It is already the sixth day and we have told the government that if it doesn't act on these issues in the next 48 to 72 hours it will be too late," he warned.

Kathmandu has witnessed an exodus of worried residents returning to rural villages since the 7.8-magnitude quake flattened large areas of the once vibrant city, some fearful that the capital will be gripped by a health epidemic.

For those who remain, the situation is grim, with families of 15 often packed into one tent and many complaining of overflowing public toilets at the makeshift camps sprawled across Tundikhel and Khula Manch, large open grounds in the city centre.

"We dread the thought of going to these toilets and now even the open ground that is left has limited space," Krishna Seva told AFP at Tundikhel Maidan.

Health workers have been out clearing rubbish and spraying disinfectants, but some left destitute complained of a lack of help from the authorities.

"We have been using our own disinfectants in our tent to try to stay disease free," said 25-year-old Gaurav Kalki, squeezed into a tent with his family and two dogs.

Fuller of the Red Cross said the priority was establishing "proper water supply points" across Kathmandu and neighbouring devastated districts.

- Rotting cattle -

"I don't want to talk up epidemics at this point but for the young and elderly their immune systems weaken substantially when they're not getting proper nutrition, clean water and a decent environment to live in," he said.

"We've talked to a lot of people and they don't have access to water.

"Organisations have been distributing bottled water and I think the government is trying to make efforts to bring water to the different areas, but this is a city of 2.5 million people and everyone is in the same boat," the spokesman added.

He said the situation was likely to be better in rural areas where villages have access to running streams.

Delhi Medical Association President Ajay Lekhi said there was also a "serious risk from rotting bodies of humans, cattle and pets".

"It's not possible to remove all the bodies at once. There will be carcasses buried inside the rubble. The risk of malaria and other mosquito-borne diseases cannot be ruled out either because of the wet and humid weather conditions," he added.

For Kalki's uncle Shahi, the uncertain future and fear of disease is as terrifying as the moment the quake struck.

"I wonder what we will do now and how much longer we will have to stay out here," he said.