Approximately 1.3 million people are currently living in shelters in the war-torn Gaza Strip, according to head of the World Health Organization, who warned Wednesday that "a sustained ceasefire" between Israel and Hamas is "a matter of life or death for civilians" in Gaza.

WHO Director-General Tedros Adhanom Ghebreyesus said "overcrowding and lack of food, water, sanitation and basic hygiene, waste management and access to medication are resulting in a high number of cases of" various contagious diseases, including 111,000 cases of acute respiratory infections, 12,000 cases of scabies, 11,000 cases of lice, 75,000 cases of diarrhoea, 24,00 cases of skin rash, 2,500 cases of impetigo, 2,500 cases of chickenpox and 1,100 cases of jaundice.

"On top of all this: heightened risk of disease outbreaks," Tedros wrote in a post on X, the social media platform formerly known as Twitter.

"Given the living conditions and lack of health care, more people could die from disease than bombings," he added. "We need a sustained ceasefire. NOW. It's a matter of life or death for civilians."

Gaza, a 140-square-mile territory governed by the Palestinian militant group Hamas, is home to more than 2 million people.

-ABC News' Morgan Winsor

The Israeli Prime Minister's Office said Wednesday that 161 hostages are still being held in the Gaza Strip.

Of the 161 remaining hostages, 126 are male and 35 are female. Among them are 146 Israelis and 15 foreigners, including Americans. Ten of them are age 75 and older, four are 18 or 19, and four are under the age of 18, according to the Israeli Prime Minister's Office.

So far, 86 hostages -- 66 Israelis and 20 foreigners -- have been freed since last Friday in exchange for the release of dozens of Palestinians from Israeli prisons as part of a cease-fire agreement between Gaza's militant rulers, Hamas, and Israel. Another prisoner swap is expected to take place Wednesday.

-ABC News' Jordana Miller, Joe Simonetti and Morgan Winsor

The United States' Special Presidential Envoy for Hostage Affairs Roger Carstens, often referred to as the country's top hostage negotiator, will travel to Israel on Wednesday, according to a senior official in the U.S. Department of State.

The official said that Carstens "will support Secretary Blinken's visit to the region, meet with Israeli government counterparts and visit the families of Americans held hostage in Gaza."

U.S. Secretary of State Antony Blinken is expected to arrive in Israel on Wednesday night.

It will be Carstens' first known trip to Israel since Oct. 7, when Palestinian militant group Hamas carried out an unprecedented attack on southern Israel and took hundreds of people hostage back to the neighboring Gaza Strip. Carsten's deputy, Steven Gillen, visited Israel shortly after the war broke out and has spent a substantial amount time in the region during the weeks that followed, largely coordinating with Israeli officials on efforts to free the hostages.

Earlier this month, Carstens met with family members of some of the hostages in Washington, D.C.

-ABC News' Shannon Crawford and Morgan Winsor

White House National Security spokesperson John Kirby said there's no indication that Hamas is trying to keep Americans as hostages in order to use them as leverage.

"There's no indication that Hamas is trying to play some sort of game here in terms of the Americans," Kirby told reporters Tuesday.

He added that the first test case of getting hostages out involved two Americans, including a teenager from the Chicago area, and that 4-year-old American-Israeli hostage Abigail Idan was released over the weekend.

In the latest exchange on Tuesday, 12 hostages -- 10 Israelis and two foreign nationals from Thailand -- were released, the Israeli government said.

Kirby reiterated that the pool of American hostages is "pretty small, and the pool of Americans that qualify right now, women and children, is smaller still."

He also noted that there are issues locating the hostages, who may be held by various groups.

The U.S. believes there are eight or nine Americans still being held in Gaza, Kirby said on Monday.

-ABC News' Selina Wang and Cheyenne Haslett

Hundreds of demonstrators gathered on Saturday at the first anti-government protest held in Tel Aviv since Oct. 7.

"We don't want a radical government. We didn't want it before, and we don't want it now," Moshe Radman, who helped organize the protest, told ABC News. "We are telling them we want a new government as soon as possible."

Radman says people were scared to protest in the early days of the war -- but that things have changed in the last few weeks. He predicts the demonstrations will grow.

"We gave them 57 days, and week after week we saw that they are not doing the right thing. And I think we understand that we are headed towards a long war, so we have to do it now," he said.

Protesters Sharon and Eyal Eshel said their 19-year-old daughter Roni, an Israel Defense Forces soldier, was killed in the Oct. 7 attack by Hamas militants while stationed at the Nahal Oz kibbutz. Her family said they have yet to receive a single phone call from Israeli Prime Minister Benjamin Netanyahu.

"We are looking for answers, we want answers from the IDF, we want answers from the government. And we want the man who is in charge, we need him to take responsibility," Sharon Eshel told ABC News.

"It's not the time to say, 'It's not the time,'" Eyal Eshel told ABC News, calling on Netanyahu to sit down with him and answer his questions. "Enough is enough."

In response to criticism and pool polling numbers, Netanyahu has previously said he has no plans of stepping down.

-ABC News' Dragana Jovanovic, Dorit Long, Kuba Kaminski and Ines De La Cuetara