Pneumonia affects the lungs of the human body. Pneumonia is an inflammatory disease of the lungs. It is an inflammation of the parenchyma of the lungs . Pneumonia is usually caused by a virus , bacteria , or fungal infection. Mild or mild pneumonia can be life-threatening. There is also the possibility of getting the flu from pneumonia. Pneumonia is more common in older people , who have had a chronic illness, or who have a weakened immune system. However, young , young , and healthy people can also get pneumonia. Infection with streptococcus-like bacteria or respiratory syncytial virus (RSV) in the lungs causes the lungs to swell , filling up with pus or fluid , which reduces the ability to breathe by taking in oxygen. Then there is inflammation in the lungs.