What Causes Pneumonia?
Many different germs can cause pneumonia. These
include different kinds of bacteria, viruses, and, less often, fungi.
Most of the time, the body filters germs out of the
air that we breathe to protect the lungs from infection. (For more information,
see the Diseases and Conditions Index
“How
the Lungs Work” article.) Sometimes, though, germs manage to enter
the lungs and cause infections. This is more likely to occur when:
- Your immune system is weak
- A germ is very strong
- Your body fails to filter germs out of the air
that you breathe
Your mouth and airways are exposed to germs as you
inhale air through your nose and mouth. Your immune system, the shape of your
nose and throat, your ability to cough, and fine, hair-like structures called
cilia (SIL-e-ah) help stop the germs from reaching your lungs.
For example, coughing is one way the body keeps
germs from reaching the lungs. Some people may not be able to cough because,
for example, they’ve had a stroke or are sedated (given medicine to make
them sleepy). This means germs may remain in the airways rather than being
coughed out.
When germs do reach your lungs, your immune system
goes into action. It sends many kinds of cells to attack the germs. These cells
cause the alveoli (air sacs) to become red and inflamed and to fill up with
fluid and pus. This causes the symptoms of pneumonia.
Germs That Can Cause Pneumonia
Bacteria
Bacteria are the most common cause of pneumonia in
adults. Some people, especially the elderly and those who are disabled, may get
bacterial pneumonia after having the flu or even a common cold.
Dozens of different types of bacteria can cause
pneumonia. Bacterial pneumonia can occur on its own or develop after you've had
a cold or the flu. This type of pneumonia often affects one lobe, or area, of a
lung. When this happens, the condition is called lobar pneumonia.
The most common cause of pneumonia in the United
States is the bacterium Streptococcus (strep-to-KOK-us)
pneumoniae, or pneumococcus (nu-mo-KOK-us).
Lobar Pneumonia
Figure A shows the location of the
lungs and airways in the body. It also shows pneumonia thats affecting
the lower lobe of the left lung. Figure B shows normal alveoli. Figure C shows
infected alveoli.
Another type of bacterial pneumonia is called
atypical pneumonia. Atypical pneumonia includes:
- Legionella pneumophila. This is
sometimes called Legionnaire's disease. This type of pneumonia has caused
serious outbreaks. Outbreaks have been linked to exposure to cooling towers,
whirlpool spas, and decorative fountains.
- Mycoplasma pneumonia. This is
a common type of pneumonia that usually affects people younger than 40. People
who live or work in crowded places like schools, homeless shelters, and prisons
are most likely to get it. It’s usually mild and responds well to
treatment with antibiotics. But, it can be very serious in some people. It may
be associated with a skin rash and hemolysis (the breakdown of red blood
cells).
- Chlamydophila pneumoniae. This kind of
pneumonia can occur all year and is often mild. The infection is most common in
people 65 to 79 years of age.
Viruses
Respiratory viruses cause up to one-third of the
pneumonia cases in the United States each year. These viruses are the most
common cause of pneumonia in children younger than 5 years.
Most cases of viral pneumonia are mild. They get
better in about 1 to 3 weeks without treatment. Some cases are more serious and
may require treatment in a hospital.
If you have viral pneumonia, you run the risk of
getting bacterial pneumonia also.
The flu virus is the most common cause of viral
pneumonia in adults. Other viruses that cause pneumonia include respiratory
syncytial virus, rhinovirus, herpes simplex virus, severe acute respiratory
syndrome (SARS), and more.
Fungi
Three types of fungi in the soil in some parts of
the United States can cause pneumonia. These fungi are coccidioidomycosis
(kok-sid-e-OY-do-mi-KO-sis) in Southern California and the desert Southwest,
histoplasmosis (HIS-to-plaz-MO-sis) in the Ohio and Mississippi River Valleys,
and cryptococcus (krip-to-KOK-us). Most people exposed to these fungi
don’t get sick, but some do and require treatment.
Serious fungal infections are most common in people
who have weak immune systems as a result of long-term use of medicines to
suppress their immune systems or having HIV/AIDS.
Pneumocystis jirovecii (nu-mo-SIS-tis ye-RO-VECH-e),
formerly Pneumocystis carinii, is sometimes considered a fungal pneumonia.
However, it’s not treated with the usual antifungal medicines. It usually
affects people who:
- Have HIV/AIDS or cancer
- Have had an organ and/or
bone
marrow transplant
- Take medicines that affect their immune systems
Other kinds of fungal infections also can lead to
pneumonia. |