Former President George H.W. Bush has been admitted to the intensive care unit of the Houston-area Methodist hospital. Bush has been in the hospital for more than a month now, battling what is reported to be a constant fever.

According to the Houston Chronicle, Bush’s fever is not responding to treatment with Tylenol. The fever rose on Wednesday, December 24, but doctors are reported to have said Bush has improved since that time. Bush is now considered to be in “guarded” condition. The cause of the fever has not been released.

Bush was the 41st president of the United States, serving one term from 1989 to 1993. Before being elected president, Bush flew 58 U.S. Navy combat missions during World War II. He went on to become the director of the Central Intelligence Agency and President Ronald Reagan’s vice president from 1981 to 1989.

The Associated Press (AP) is reporting that Bush was visited on Christmas by his wife, former First Lady Barbara Bush, and his son Neil’s family. The other members of Bush’s family have also been able to visit him in the past month, including former U.S. president George W. Bush, and former Florida Governor Jeb Bush.

Despite being 88 years old, Bush was known to be healthy and active in recent years. He celebrated his 85th birthday, just three years ago, by jumping out of an airplane: