Pneumonia is spread from person to person when a person with the infection coughs or sneezes and another person breathes in those germs. The germs that cause pneumonia enter the person’s airways and multiply, developing the infection in the lungs. Another way pneumonia is spread is when bacteria and viruses that normally are in your nose and mouth accidentally enter your lungs. When your body is sleeping, it is common that saliva will move from the mouth, nose and throat – normally the body will cough back up the secretions and your immune system will prevent the organisms from causing pneumonia. However, people with weak immune systems or those that are ill can develop a severe cause of pneumonia when the secretions enter the lungs. Once the organisms enter the lungs they settle in the air sacs of the lungs and they quickly grow in number. This area of the lung becomes filled with fluid and pus (thick, yellow fluid) as the body tries to fight the infection.