A month after the exodus began, those dispersed in some 68 camps and settlements along the border do not have safe drinking water and hygiene facilities, the WHO cautioned.
The camps also face dire shortages of food and medicine in what has quickly become one of the world's largest refugee settlements.
"Risk of water borne diseases is high, especially there is very high risk of cholera and this is why everyone is concerned," the WHO said in a statement.
"Interventions are being scaled-up, however, the situation remains critical and challenging."
The latest influx has overwhelmed the camps around Cox's Bazar, which previously housed at least 300,000 people who had fled earlier violence in Myanmar's Rakhine state.
The WHO said mobile medical centers have been set up, while Bangladesh health authorities say they have treated some 4,500 Rohingya for diarrhea in a month and vaccinated some 80,000 children for measles and polio.
"We are trying our best to face the challenges. But we are concerned," Enayet Hossain, deputy head of Bangladesh's health services department, told AFP.
The department said that at least 10 Rohingya have died in Bangladesh since the influx, most from bullet and blast wounds suffered in Rakhine.
Two elderly Rohingya men died of diarrhea at a charity clinic more than a week ago, said Misbah Uddin Ahmed, a health department official at Ukhia, where most of the camps are located.
More than 436,000 refugees have crossed the border from Rakhine since August 25 when a military crackdown was launched following attacks by Rohingya militants, according to UN figures Monday.
The refugees have given shocking accounts of killings and mass rapes by Myanmar soldiers and Buddhist mobs. The Myanmar military says they have only targeted Rohingya militants.