Bronchitis can escalate to pneumonia if you do not get treatment. Bronchitis is a lung illness caused by an inflammation of the airways leading to the lungs. Pneumonia is a lung illness that can affect one or both lungs. If untreated bronchitis is not treated, the infection can spread from the airways to the lungs. This second stage of the disease is called "acute bronchitis turning to pneumonia." Pneumonia is more serious than bronchitis. With pneumonia, your respiratory system is affected by bacteria or viruses. The virus or bacteria may come from someone else or the organism may be present in the environment. For example, bacteria that cause pneumonia are found in the air and water around us. When you breathe in these organisms, they can settle in your lungs where they can grow and infect cells. People often think that pneumonia means full-body pain. In fact, the body's defense mechanism is responsible for any pain you feel during pneumonia. Your immune system is trying to fight off the invading organism. Thus, when you have pneumonia, you experience pain because of this powerful response by your body. The most common cause of pneumonia is Streptococcus pneumoniae (the pneumococcal strain). This bacteria can be found in the nose, throat, and stomach of humans. When someone with pneumococcal pneumonia eats or drinks something contaminated with the bacteria, they can pass it on to others.