According to the World Health Organization, if the health system is not repaired, untreated illnesses would eventually kill more people in Gaza than bombs.
Diarrhoea and respiratory infections are widespread among children in overcrowded UN facilities where almost 1.1 million people are sheltering.
Patients with chronic illnesses like cancer are also receiving no treatment.
The warning comes as a truce between Israel and Hamas entered a fifth day, after a 48-hour extension was agreed.
The deal mediated by Qatar, Egypt and the United States should see an additional 20 Israeli women and children held hostage in Gaza being released in exchange for 60 Palestinian women and teenagers in Israeli prisons.
On Monday, 11 hostages and 33 prisoners were freed on the fourth and final day of the initial agreement, bringing the totals to 50 hostages and 150 prisoners released.
Nineteen foreign nationals, one of whom has Israeli citizenship, have also been handed over by Hamas under separate agreements.
Israel launched a military campaign in Gaza and imposed a siege in response to an unprecedented cross-border attack by Hamas gunmen on 7 October, in which at least 1,200 people were killed and about 240 others taken hostage.
Gaza’s Hamas-run government says more than 14,800 people have been killed in the territory since the war began.
