Preventing outbreaks of diseases across tsunami-hit Asia is a race against time, but contrary to popular belief, the thousands of rotting corpses do not pose the main threat, health officials said on Thursday.

Sewage-contaminated water is the main risk factor in the spread of deadly diseases like cholera, malaria and dengue fever, and aid organizations are scrambling to get clean drinking water to the affected areas.

“There is no risk of epidemics because of dead bodies,” said Harsaran Pandey, World Health Organization (WHO) regional spokeswoman in New Delhi.

Most agents carrying diseases do not survive long in the human body after death and the source of acute infections was more likely to be tsunami survivors who may already be carrying diseases, the WHO said.

The giant walls of water that crashed ashore on Sunday destroyed safe water supplies for hundreds of villages and polluted what was left with sewage. It is this contaminated water that poses the greatest threat, the WHO said.

The international health organization has said the tsunami death toll, now at more than 87,000, could double if epidemics broke out in the affected countries.

“I could say many, many, many people could die if we are not able to reach people in a timely manner with safe water for the prevention of death from water-borne diseases,” said Pandey.

“Diseases are spread by contaminated water, drinking water and water used to clean food,” she said.

The first symptom of contaminated water consumption is diarrhoea and doctors in Sri Lanka and elsewhere are already reporting cases of diarrhoea and vomiting.

“If a person gets dehydrated from diarrhoea then that is when it gets serious and a person can die,” Pandey said.

If deadly diseases are present, people will start showing symptoms within a few more days, she said.

The WHO and other international aid bodies are shipping tons of water purification tablets to Indonesia, Sri Lanka and the Maldives, while foreign navies are sending ships with desalination units to produce drinking water.

However, while rotting corpses pose little threat to survivors, relief workers handling them face a risk of contracting tuberculosis, hepatitis, HIV, as well as gastrointestinal infections such as diarrhoea, salmonellosis, typhoid fevers and cholera.

Wide tracts of coastal land in Indonesia, Thailand, India and Sri Lanka have been flooded by the tsunami and this may lead to an increase in mosquito-borne diseases such as malaria and dengue fever, which are endemic in Southeast Asia.

The WHO said flooding may initially flush out mosquito breeding grounds, but once the waters recede in six to eight weeks, people risk outbreaks of malaria.

The U.N. children’s organization UNICEF has said a third of tsunami victims may be children and they are the most vulnerable to infections such as pneumonia and measles.

“You would worry in the next stage for the children,” said Pandey.