It depends. Some pathogens such as tuberculosis are dangerous to everyone. Other causes of pneumonia only infect people with very weak immune systems. On a daily basis, various pathogens are inhaled or aspirated many times. However, a healthy immune system (is) provides barriers & cell defense systems which prevent pneumonia. If there is a temporary weakness in ones is then pneumonia may develop.
It depends. Infection within the lungs (pneumonia) involves 2 basic principles: an infectious pathogen (bacteria, virus, fungus or parasite) and a failure of the immune system. Most pathogens enter the lungs from simple breathing. They may be carried in the air or within very tiny droplets. Occasionally pathogens are aspirated into the lungs from the mouth or stomach during choking. (see additional).