Pneumonia is an inflammation of the lung that is most often caused by infection with bacteria, viruses, or other organisms. Occasionally, inhaled chemicals that irritate the lungs can cause pneumonia. Healthy people can usually fight off pneumonia infections. However, people who are sick, including those who are recovering from the flu (influenza) or an upper respiratory illness, have weakened immune systems that make it easier for bacteria to grow in their lungs. Community-Acquired Pneumonia (CAP). People with this type of pneumonia contracted the infection outside a hospital setting. It is one of the most common infectious diseases. It often follows a viral respiratory infection, such as the flu. One of the most common causes of bacterial CAP is Streptococcus pneumoniae.