And the devastation wrought by the conflict there made coping with the outbreak that much more difficult, WHO spokesman Tarik Jasarevic told reporters in Geneva.
"The number of suspected cholera cases is rising," said Jasarevic.
"We are talking close to 2,000 suspected cases per day," he warned, which meant that Yemen was now suffering the world's largest cholera outbreak.
Since WHO began collecting data on the outbreak on April 27, it has registered more than 170,000 suspected cholera cases across 20 of Yemen's 21 governorates, Jasarevic said.
WHO has warned that a quarter of a million people could fall sick with the disease cholera by the end of the year there. Already, two-thirds of the population are on the brink of famine there.
Cholera is a highly contagious bacterial infection spread through contaminated food or water. The disease is easily treatable, but reining it in conflict-torn Yemen is particularly difficult.
Two years of devastating war between the Huthis and government forces backed by a Saudi-led Arab military coalition has killed more than 8,000 people, and wounded 45,000 others.
But it has also devastated the country's infrastructure, leaving more than half the country's medical facilities out of service.
"It is difficult in a situation where a country has a health system that is collapsing," Jasarevic said.
The WHO and other UN agencies and aid organisations were trying to scale up their response, he added.
WHO has so far provided more than 220,000 bags of intravenous fluid, has established 144 diarrhoea treatment centres, 206 oral rehydration therapy corners, and nearly 2,000 beds for the treatment of cholera patients, he said.
Yemen's healthcare workers have not received salaries for months, Jasarevic added. So WHO and UNICEF had begun paying incentives to some doctors and nurses to discourage them from demanding payment from patients who cannot afford it.
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