Senator Robert C. Byrd, left, was taken to a hospital Monday evening and will be treated there for a mild infection for several more days, his office said. Mr. Byrd, 90, a West Virginia Democrat who is the longest-serving senator, was hospitalized after feeling lethargic at work. He went home after a 5:30 p.m. Senate vote, was found to have a fever and was taken to Inova Fairfax Hospital for observation. A spokesman, Jesse Jacobs, said Mr. Byrd was “alert, talking with staff and in good spirits, and looks forward to getting back to work as soon as possible.”.