After feeling Kelly's swollen lymph nodes in her neck, Dr. Bancroft explained that they were swollen because her body was fighting an infection.
Although she hasn't yet gotten the infection or felt it kicking in, her body is nevertheless reacting to the impending infection that is about to enter her body. This is why her lymph nodes are swelling up, because they are trying to produce materials that will help her body eradicate this infection even before it actually infects her body.
