If it is about what happens during an attack of pneumonia then as much I know, Patient feels as if he can't breath, he feels some strange feeling in his chest. When ever he tries to take air inside his lungs, his lungs quickly contract and air come out again. And if you quickly do not heat his body, then he may die for the lack of breath or oxygen.
During viral pneumonia, a patient will get some symptoms like coughing, fever, shaking chills and shortness of breath. You should visit a doctor to have a complete physical exam and a chest x-ray. For more detailed information, go to http://www.nlm.nih.gov/medlineplus/ency/article/000073.htm.