What causes pneumonia?

Pneumonia is an infection of the lungs. It is usually caused by bacteria, but can also be caused by viruses, fungi, or parasites. The most common bacterial causes of pneumonia are Streptococcus pneumoniae, Haemophilus influenzae, and Mycoplasma pneumoniae. The most common viral causes of pneumonia are influenza viruses, respiratory syncytial virus, and adenoviruses.

Risk factors for pneumonia include:

* Young age (less than 2 years old)

* Old age (65 years or older)

* Chronic medical conditions, such as heart disease, diabetes, or chronic lung disease

* Smoking

* Alcoholism

* Poor nutrition

* Living in crowded conditions

* Exposure to environmental toxins, such as air pollution or chemicals

Pneumonia can also be spread through contact with an infected person, by inhaling droplets of respiratory secretions that contain the bacteria or virus that cause pneumonia.