The virus that causes polio has been detected in sewage samples in Gaza, according to its health ministry.
It said tests had been carried out with UNICEF and that the finding “exposes thousands of residents to the risk of contracting poliomyelitis”.
It warned of a potential “new health disaster” if action isn’t taken.
Polio mainly affects children and usually spreads through contact with faeces; for example when people don’t wash their hands properly, or from contaminated food or water.
The dire conditions in Gaza, where infrastructure has been decimated and thousands are packed together in camps, make transmission far more likely.
In a statement, the health ministry called “for an immediate halt to the Israeli aggression, providing usable water, repairing sewage lines, and ending population crowding in places of displacement”.
In rare cases, polio can cause serious and long-term symptoms affecting the brain and nerves, including paralysis, and can sometimes be life-threatening.
However, symptoms are normally mild and flu-like.
The risk of catching it is much higher if people haven’t had a polio vaccine – a routine event in the UK and most developed countries.
Doctors in Gaza also worry there could be a cholera outbreak due to the unsanitary living conditions the war has caused.
Israel‘s campaign to wipe out Hamas continues after its terror attack killed around 1,200 people last October.
More than 38,600 people in Gaza, including many women and children, have been killed since then, according to the health ministry.
Israel says it takes the utmost care to avoid civilian casualties but that Hamas fighters deliberately embed themselves with the population.
Most of Gaza’s 2.3 million people are now in tent camps with scant food and water.
A recent strike that hit one of them – and reportedly killed at least 90 people as Israel hunted senior Hamas figures – was a vivid reminder of the dangers.
Israeli negotiators were in Egypt on Wednesday as long-running efforts to agree a ceasefire continue.
An agreement would likely see an extended pause in the fighting for the release of hostages.
Israel said earlier this month that 116 of those taken during the attack on Israel remained unaccounted for.