She first appears when Sam and Dean bring in the body of Mr. Tanner, and get treatment for Mrs. Tanner. Dr. Lee treats her and asks her what happened. During her investigation of Mr. Tanner, Dr. Lee finds a virus, more specially, sulfur in his blood. Then Dr. Lee attempts to draw blood only to be attack by Mrs. Tanner. She determines this to be the cause, and it takes 3-4 hours for the change to start. Also that it spreads from blood to blood contact.
As Sam and Dean prepare bombs in order to help them escape, Duane Tanner arrives, only to be tied up. The Doctor checks his blood for any signs of the virus. 3 Hours later, the doctor can't find any signs and unties Duane. She also checks on Sam who was attacked by Pam. Thinking Sam is infected, Dean tells them all to leave, but the doctor comes back to show them everyone has vanished. She later confirms that Sam is not infected with the virus. As the Winchesters and everyone else are leaving, Dr. Lee decides to stay and talk to the police.
The demon possessing Duane Turner tells Azazel that nothing will be left behind, so presumably, he went back and killed all the survivors, Amanda included.