15.4B: Viral Pneumonia
- Outline the route of infection for a virus that causes pneumonia
Pneumonia is an inflammatory condition of the lung that particularly affects microscopic air sacs (alveoli). It is associated with fever and chest symptoms, and it appears as a lack of space on a chest x-ray. The inflammation may be caused by infection from viruses, bacteria, or other microorganisms. Less commonly, it is caused by certain drugs and other conditions. Viruses and bacteria are the two leading causes of pneumonia, while fungi and parasites are less common. Viruses are the most common cause of pneumonia in children, while bacteria are the most common cause in adults.
How Viruses Cause Pneumonia
Many types of viral infections can cause pneumonia, but in order to do this, these viruses must first invade cells in order to reproduce. Typically, a virus will reach the lungs by traveling in droplets through the mouth and nose during inhalation. Once there, the virus will invade the cells that line the airways and the alveoli. This invasion often leads to cell death, in which either the virus directly kills the cell, or the cell self-destructs through apoptosis. Further damage to the lungs occurs when the immune system responds to the infection. White blood cells, in particular lymphocytes, are responsible for activating a variety of chemicals (cytokines) which cause fluid to leak into the alveoli. The combination of cellular destruction and fluid-filled alveoli interrupts the transportation of oxygen into the bloodstream. Thus, in large part, as with other viral infections, it is the body’s response to the virus that causes the symptoms of pneumonia, and not necessarily the viral infection itself. In addition to their effects on the lungs, many viruses affect other organs and can lead to illnesses that affect other bodily functions. Viruses also make the body more susceptible to bacterial infection. For this reason, bacterial pneumonia often complicates viral pneumonia.
Which Viruses Cause Pneumonia
Common viruses that cause pneumonia include influenza viruses A and B, respiratory syncytial viruses (RSV), and human parainfluenza viruses (hPIV), the last of which particularly affects children. Rarer viruses that commonly cause pneumonia include adenoviruses (in military recruits), metapneumoviruses, and severe acute respiratory syndrome virus (SARS coronavirus). Viruses that primarily cause other diseases, but sometimes cause pneumonia, include herpes simplex virus (HSV, mainly in newborns), varicella- zoster virus (VZV), measles virus, rubella virus, and cytomegalovirus (CMV, mainly in people with immune system problems). In children with pneumonia, the most commonly identified agents are respiratory syncytial virus, rhinovirus, human metapneumovirus, human bocavirus, and parainfluenza viruses. Because of this, the best prevention against viral pneumonia is vaccination against influenza, adenovirus, chickenpox, herpes zoster, measles, and rubella.
Key Points
- Viral pneumonia is caused by both viral infection which leads to cell death. The body’s response to clear the cellular debris leads to further inflammation and the blockage of respiration.
- Many different viruses can cause viral pneumonia, but they all enter the lungs and damage the alveoli.
- The best prevention for viral pneumonia is to vaccinate against the viruses that can cause pneumonia.
Key Terms
- Alveoli : alveolus (plural alveoli) a small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood.
- cytokines : Regulatory proteins that function in the regulation of the cells involved in immune system function
- apoptosis : The process of programmed cell death by which cells undergo an ordered sequence of events which lead to death of the cell. This occurs during growth and development of the organism, as a part of normal cell aging, or as a response to cellular injury.