Dr. Khaled El Serr last spoke to his family in mid-March, a week before Israeli troops raided the hospital in southern Gaza where he worked as a surgeon.

“No one has seen or heard of him ever again,” said his cousin, Osaid AlSerr, a surgical resident in the United States. “We do not even know whether he is dead or alive.”

Dr. El Serr was arrested by the Israeli military, according to Amnesty International, citing the accounts of co-workers and Palestinian detainees who have been released. But the military has refused to say whether it is holding him.

His story is not unique. More than 300 of Gaza’s health workers are in Israeli detention, the enclave’s health ministry says, while others have been detained for a time and then released. And according to the World Health Organization, 500 have been killed in the war, out of a prewar total of about 20,000.

Based on estimates of the war’s toll, that means medical workers have been killed and detained at higher rates than Gazans generally, a severe blow to a health care system whose facilities have been devastated by war, and a population weakened by hunger, lack of clean water and the rampant spread of diseases.

“That equates to an average of two health care workers killed every day, with one in every 40 health care workers, or 2.5 percent of Gaza’s health care work force, now dead,” Medical Aid for Palestinians, a British charity, said in a statement.