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Risk Factors and Causes


Dr. Green
The majority of lower respiratory tract infections are caused by bacteria, but viruses and fungi can also invade and infect the mucous membrane of the lower respiratory tract. Captain Miller, can you tell us more about what causes a lower respiratory infection?

Captain Miller
Certainly, Dr. Green. Bronchitis, or infection of the bronchi, often starts as an upper respiratory infection that eventually spreads to the lower respiratory tract. Other times, the bronchi become inflamed due to other systemic, bacterial, or viral infections, such as:

  • Influenza
  • Rubeola, or measles
  • Rubella
  • Pertussis, or whooping cough
  • Scarlet fever, or
  • Typhoid fever

Bronchiolitis, or infection of the bronchioles, typically affects infants, and is most often caused by respiratory syncytial virus, or RSV. Other viruses can also cause bronchiolitis.

There are a number of factors, including environmental contaminants, autoimmune diseases, and infections, that can cause pneumonitis, or inflammation of the lungs. When inflammation of the lungs is caused by infection, it's called pneumonia. Most infections that cause pneumonia are bacterial infections, but viruses and fungi can also cause pneumonia, especially in the very young, the very old, and people with weakened immune systems.

Bacteria, viruses, and fungi that infect the lower respiratory tract may spread through tiny air droplets that are released when an infected person sneezes, coughs, or talks. Other times, infection sets in when the bacteria that normally live in the upper respiratory tract, called flora, get into the lungs.

Normally, nasal hairs and wet mucus trap foreign particles and microbes to protect the respiratory tract. In addition, the adenoids and tonsils, which are located in the pharynx, or throat, release white blood cells and antibodies that fight and destroy invading microbes.

However, many viruses and bacteria are able to adapt in order to resist destruction. For example, some microbes produce toxins that impair the body's natural defenses. Others change their shape or outer structural proteins to disguise themselves from being recognized by the immune system.

Infection occurs when an invading virus, bacterium, or fungus overcomes the body's natural defenses and invades the mucous membrane, or inner lining, of the respiratory tract.