During the movie, Lexi breaks the window with her mobile phone, and Brad does a good job of resealing the window. Too good a job in fact. In turns out that Lexi let in a bit of the bacteria but since the house was sealed well making the house very warm, the bacteria used the house as a breeding ground and Brad became a "lethal carrier" of a new strain of virus that has no cure.The irony is, the virus released by the bombs attacks in large quantities. Lexi survives because she spent most her time outside and Brad is killed (via gassing) in his house as there is no cure for him. Had Brad not sealed up the house and allowed for some air to get in, he may have survived.