The highly infectious polio virus has been found in sewage samples in Gaza, putting thousands of Palestinians at risk of contracting a disease that can cause paralysis.

Gaza’s Ministry of Health and the World Health Organization (WHO) both said they had carried out tests and found samples of the virus in sewage water.

“Poliovirus type 2 (VDPV2) had been identified at six locations in sewage samples collected on 23 June from Khan Younis and Deir al Balah,” WHO said Friday.

WHO said the findings are linked to the “disastrous sanitation situation” created by Israel’s brutal military assault in Gaza since the Hamas attacks of October 7.

“It is important to note the virus has been isolated from the environment only at this time; no associated paralytic cases have been detected,” WHO added. It said no one has yet been treated in Gaza for paralysis or other symptoms of polio, but that residents must now “contend with the threat” posed by the disease.

Various United Nations agencies – including UNICEF, the children’s fund, and UNRWA, the agency for Palestinian refugees – are working with local health authorities to determine how far the virus has spread.

WHO chief Tedros Adhanom Ghebreyesus said polio vaccination rates prior to the conflict were “optimal,” but that Israel’s war against Hamas had created “the perfect environment for diseases like polio to spread.”

“The decimation of the health system, lack of security, access obstruction, constant population displacement, shortages of medical supplies, poor quality of water and weakened sanitation are increasing the risk of vaccine-preventable diseases, including polio,” Tedros warned.

The Palestinian Ministry of Health in Gaza called for practices to improve hygiene and safety.

“Detecting the virus that causes polio in sewage portends a real health disaster and exposes thousands of residents to the risk of contracting polio,” it said in a statement, demanding “an immediate halt to the Israeli aggression.”

Wild polio was eradicated from Gaza more than 25 years ago, with pre-war vaccination coverage reaching 95% in 2022, according to WHO.

Poliovirus can emerge when poor vaccination coverage allows the weakened form of the orally administered vaccine virus strain to mutate into a stronger version capable of causing paralysis, a spokesman from WHO’s global Polio Eradication program said.