GENEVA (Reuters) - Hundreds of thousands of Rohingya refugees who fled from Myanmar into Bangladesh are at risk from a possible outbreak of diphtheria, the World Health Organization said in a report published on Tuesday.

More than 620,000 people from Myanmar’s Muslim Rohingya minority have fled to Bangladesh since the end of August and are camped in squalor after escaping a military crackdown the United States said included “horrendous atrocities” aimed at “ethnic cleansing”.

In a weekly roundup of illness among the displaced Rohingya population, totaling about 820,000, the WHO said a 30-year-old woman was found to have suspected diphtheria on Nov. 10, but she disappeared during treatment two days later.

WHO investigators failed to find her, but on Nov. 19 a man in the same area, the Balukhali makeshift settlement, was also reported to have suspected diphtheria, and another member of his household was sick.

Tests on throat swabs are still pending, the report said.

The WHO describes diphtheria as a widespread, severe infectious disease that has the potential for epidemics, with a mortality rate of up to 10 percent.

It is caused by bacteria that produces a toxin that can harm tissues and organs. Treatment involves administering anti-toxin to neutralize the toxin and antibiotics to kill the bacteria.

Many of the refugees were not vaccinated against diphtheria as children so there is little immunity against the disease.

“Therefore, obstacles to optimal vaccine delivery must be identified and forceful measures taken to improve immunization coverage immediately,” the report said.

After the flood of migrants began, health officials rushed to vaccinate against cholera, averting what was seen as the greatest single risk. When measles broke out, causing 1,270 suspected cases, they rapidly vaccinated 135,000 children against measles and rubella.

But conditions are still dire, and testing of water supplies between Sept. 18 and Nov. 14 showed 82 percent of water tested positive for faecal contamination with E.coli bacteria. Between Aug. 25 and Nov. 18, there were 45,790 cases of acute watery diarrhea and 11 deaths.

There have been 190 cases of acute jaundice syndrome and risk of a hepatitis E outbreak, which can kill 30 percent of pregnant women who catch it.