Because Lexi (Mary McCormack) broke the glass of one of the doors, she allowed a small amount of the toxic ash into the house. As a result, and because Brad (Rory Cochrane) did such a good job of sealing up the non-airconditioned house, the temperature and conditions inside the house turned it into an incubator for the virus. This turned the toxin into a highly lethal strain and made Brad a carrier. The army take away Lexi for treatment and seal up their home with him still in it. With a fumigation tent over the house they begin to pump in a toxic gas, killing Brad. It is implied that because Brad did not let Lexi into the house he saved her life, as she recieves treatment for her symptoms.