Pneumonia is a serious infection of the lungs that is the 6th leading cause of death in the United States. Thanks to the development of antibiotics, a third of the population no longer die from it. Pneumonia is caused by breathing in the organisms that cause Pneumonia, which can happen when somebody carrying the organisms coughs or sneezes. It can also be cuased when bacteria or viruses in the mouth, throat, or nose secrete into the lungs. Considering that the Longchamps don't have a lot of money, it's a good thing Jimmy did not get Pneumonia, though it's not as likely for him to get it as Dawn thinks.