Monday night, Sen. Robert C. Byrd was taken to hospital and remained under observation as he was feeling lethargic and had a fever.

According to The Associated Press, a spokesman said that his caregiver was concerned about the senator’s condition and took his temperature. When she noticed that he had a fever, she announced Byrd’s doctor.

The doctor recommended that he be taken to hospital and be held there overnight for observation. Press secretary Jesse L. Jacobs said that he didn’t know the hospital where Byrd was taken.

Earlier in the day, Byrd was at the Senate voting on global warming. After the vote, he went home.

Senator Byrd is the oldest member of the Senate. He is 90-years-old.

The senator was hospitalized March 5, after he had a reaction to antibiotics and, a week before that, he was taken to Walter Reed Army Medical Center after he fell down at his home.

The West Virginia senator was one of the leading Democrats in Congress who opposed the 2003 U.S. attack on Iraq.

He is the chairman of the Senate Appropriations Committee and the longest-serving senator in history.

The first time he won the election to the Senate was in 1958.