Louis stays encouraged with the vaccine and discovers that weaker or aged viruses take longer to attack. Joseph's mother begs him to use his untested treatment on her son to save him, but Pasteur knows he could be executed and imprisoned.
Louis concludes that he must try and save the child, so he applies his vaccination to the child. Dr. Zaranoff comes from Russia with a bunch of people with rabies, offering to test his treatment as well.
Annette goes into labor with Dr. Martel's child. Frantically, he finds Dr. Charbonnet and begs him to deliver his daughter's child. He agrees, and Louis makes sure that Charbonnet stays sterile before working with Annette.
Louis' wife, Marie, finds Louis to have had a stroke. He goes into a coma for 10 days and then awakens.
Although Pasteur is still ill and recovering, he receives word that he is allowed to treat the Russian people who have not died. He orders Marie to help him get dressed. In a wheelchair he treats the ill with his vaccination, which is a success.
Louis' wife tells him that Dr. Lister is taking credit for his accomplishments at the Academy of Medicine so, he angrily goes over to scold Lister and announce his vaccines were his. It was just a big surprise for him. He is greeted by applause and recognition.