Montag took part in a large fire, which ended in the burning of thousands of books and the death of the woman who owned the house. He also stole a book and was filled with guilt. On top of that...... he learned about Clarisse's death from Millie. During the night, Montag contracted a fever..... maybe the guilt..... the shock.... or simply the stress. None-the-less, he woke up with flu like symptoms.
He had chills and fever in the morning.
"You can't be sick," said Mildred. He closed his eyes over the hotness. "Yes."