Pneumonia is caused by viruses, bacteria, or parasites (in rare cases) or other micro-organisms. In up to 65% of cases, the organism (for example bacteria or virus) which is causing Pneumonia is not identified even with testing. In adults, Pneumonia is most often caused by bacteria, for example Streptococcus Pneumoniae, Staphylococcus aureus, Haemophilus influenzae and Legionella. The Legionella bacteria was the cause of most-known outbreak of Pneumonia called Legionnairesí disease. Young children are more susceptible to develop Pneumonia from exposure to a virus, for example the parainfluenza and influenza viruses, adenovirus, and respiratory syncytial virus. The chickenpox virus also causes Pneumonia in adults and children. Mycoplasma Pneumoniae causes the symptom-less or walking Pneumonia, which mostly and usually affects older children and young adults. Dried bird-droppings contain Chlamydia psittaci which when inhaled causes respiratory problems and lead to Pneumonia. Pneumocystis carinii, which is classified as a parasite as well as fungi, causes Pneumonia in people with compromised immune system, for example, those with AIDS or undergoing cancer treatment. Healthy lungs sans have bacteria or virus owing to the guard system of our respiratory system that helps keep-out foreign organisms. However, these guards will be overwhelmed when exposed to a large number of the organisms that cause Pneumonia.
Pneumonia usually starts when a person inhales infected air particles into the lungs. In other cases, it develops during or after a viral upper respiratory infection, such as a cold or the flu. Pneumonia can even occur as a complication of viral illnesses such as measles and chickenpox. Pneumonia can also develop if a person inhales infected food, vomit, or mucus (aspiration pneumonia). Some cases of pneumonia are contracted by inhaling disease-causing microbes, e.g. bacteria or virus, which infect the lungs and cause pneumonia. When an infected person sneezes or coughs, these microbes become airborne again and infect more people. And in few cases, pneumonia is caused when some mouth, throat or nose dwelling bacteria or virus enter the lungs, causing inflammation and infection. When sleeping it is quite common for people to aspirate secretions from the nose, mouth or throat. Normally, the bodyís immune system and reflex response (coughing back up the secretions) will prevent a pneumonia from gaining. However, if a personís immune system is weakened owing to another illness, a severe pneumonia can develop. People with emphysema, heart disease and swallowing problems, alcoholics and drug abusers, and people who have suffered from a stroke or seizure are more at high risk for developing pneumonia.
Once bacteria, virus, or fungi enter the lungs, they settle in the air sacs of the lung where they rapidly multiply and grow in number. As the body attempts to fight off the infection, these air sacs of the lungs fills up with fluids and pus. Bacterial pneumonias are more serious and are the most common cause for pneumonia in adults, especially Streptococcus pneumoniae (pneumococcus). Mostly, respiratory viruses are the main cause of pneumonia in young children, most prevalent between ages of 2 and 3. By school age, Mycoplasma pneumoniae becomes the most common cause. In some people, especially the elderly and those who are debilitated, bacterial pneumonia may follow flu or even a common cold. Many people contract pneumonia when staying in a hospital for other treatments. This is more serious because the patient's immune system is impaired owing to the condition that he/she suffered previously, which required treatment. In addition to all this, there is also a greater possibility of infection with antibiotic-resistant bacteria.