Senator Robert C. Byrd, left, Democrat of West Virginia, was released from the hospital after being treated for a mild infection. Mr. Byrd, 90, was admitted Monday night after he began feeling ill at home. He had cast a vote on the Senate floor that day and started running a fever and feeling lethargic. “Senator Byrd will complete the course of his antibiotic treatment as prescribed by his doctors at his home and is expected to return to his official Senate duties upon his doctors’ approval,” said a spokesman, Jesse Jacobs. Mr. Byrd, the longest-serving senator in history, was hospitalized twice earlier this year.