This answer can be found in the first 20 pages of the book, so if you haven't given the book a shot yet, go for it! You'll get to the answer really quickly. To help you out, Mildred, Montag's wife, is a very unhappy person underneath her exterior. Although in most of the books she seems "normal," and in fact quite happy with her friends, her t.v. shows, and with the way things are running, the first time that we meet her it is revealed just how miserable she is. When Montag comes home from work at the beginning of the book, Mildred is lying on the bed barely breathing, "not caring whehter [her breath] came or went, went or came." Before Montag even sees her his foot hits and "object on the floor" which turns out to be a bottle of sleeping pills, completely empty. Mildred had taken all of them, in a possible suicide attempt, and was now barely clinging to life.
So, Montag has to call in the paramedics who have a snake-like implement that they insert into Mildred that drains all of her tainted blood and replaces it with new, fresh blood. It's an interesting device that Bradbury goes into quite a lot of detail to describe. It also gets rid of her blood, which was dosed and damaged with the sleeping pills, and replaces it with new blood. She wakes the next morning denying anything had happened, and with no recollection of the events.
I hope that those thoughts help--give reading the book a try too, and with my explanations it should be all be pretty understandable. Good luck!
1. Mildred required emergency service because she was intoxicated with pills. Montag called the paramedics and they brought two machines whose job was to clean her stomach of any remaining or in this case the pills the machines were supposed to clean the stomach.