Dan came to see Melinda and her mother insisted he fully decontaminate himself. He finally got to see Melinda, but she started suffering from anaphylactic shock. He called for her mother, who thought he was responsible.
After Chase noted during the environmental scan that there were broken branches on the tree outside of Melinda's window, House questioned Dan about whether he had been in Melinda's room before that morning. Dan finally admitted that he had been and that he and Melinda had had unprotected sex. He was terrified her mother would find out and ban him from the house altogether. He had taken penicillin before he saw Melinda as a precaution, but it is found in semen and Melinda was allergic to it.
However, they tested Dan's semen and there was no trace of penicillin in it.