Causes:

Bacteria are the most common causes of pneumonia. However, pneumonia can also be caused by viruses, fungi, and other agents. It is often impossible to identify the specific culprit.

Many bacteria are grouped into one of two large categories by the laboratory procedure used to look at them under a microscope. The procedure is known as Gram staining. Bacteria are stained with special dyes, then washed in a special solution. The color of the bacteria after washing determines whether they are Gram-negative or Gram-positive. Knowing which group the bacteria belong to helps determine the severity of the disease, and how to treat it. Different bacteria are treated with different drugs.

Gram-Positive Bacteria. These bacteria appear blue on the stain and are the most common organisms that cause pneumonia. They include:

• Streptococcus (S.) pneumoniae (also called pneumococcus), the most common cause of pneumonia. This Gram-positive bacterium causes 20 - 60% of all community-acquired bacterial pneumonia (CAP) in adults. Studies also suggest it causes 13 - 38% of CAP in children.
• Staphylococcus (S.) aureus, the other major Gram-positive bacterium responsible for pneumonia, causes about 2% of CAP and 10 - 15% of hospital-acquired pneumonias. It is the organism most often associated with viral influenza, and can develop about 5 days after the start of flu symptoms. Pneumonia from S. aureus most often occurs in people with weakened immune systems, very young children, hospitalized patients, and drug abusers who use needles. It is uncommon in healthy adults.
• Streptococcus pyogenes or Group A streptococcus.

Gram-Negative Bacteria. These bacteria stain pink. Gram-negative bacteria commonly cause infections in hospitalized or nursing home patients, children with cystic fibrosis, and people with chronic lung conditions.

• Haemophilus (H.) influenzae is the second most common organism causing community-acquired pneumonia, accounting for 3 - 10% of all cases. It generally occurs in patients with chronic lung disease, older people, and alcoholics.
• Klebsiella (K.) pneumoniae may be responsible for pneumonia in alcoholics and other people who are physically debilitated. It is also associated with recent use of very strong antibiotics.
• Pseudomonas (P.) aeruginosa is a major cause of hospital-acquired pneumonia (nosocomial pneumonia). It is a common cause of pneumonia in patients with chronic or severe lung disease.
• Moraxella (M.) catarrhalis is found in everyone's nose and mouth. Experts have identified this bacterium as an uncommon cause of certain pneumonias, particularly in people with lung problems such as asthma or emphysema.
• Neisseria (N.) meningitidis is one of the most common causes of meningitis (central nervous system infection). The organism has also been reported in pneumonia, particularly in epidemics of military recruits.
• Other Gram-negative bacteria that cause pneumonia include E. coli, proteus (found in damaged lung tissue), enterobacter, and acetinobacter.

Atypical Pneumonia

Atypical pneumonias produce mild symptoms and a dry cough. Organisms that cause atypical pneumonias include:

• Mycoplasma (M.) pneumoniae, the most common atypical pneumonia organism. Mycoplasma is a very small bacterium that lacks a cell wall. Pneumonia caused by M. pneumoniae spreads when someone carrying the infection comes in close contact with others for a long period of time. It is most often found in school-aged children and young adults. The condition, commonly called "walking pneumonia," is usually mild.
• Chlamydia (C.) pneumoniae is now thought to cause 10% of all CAP cases. This atypical pneumonia is most common in young adults and children, and is usually mild. It is less common, but usually more severe, in the elderly.
• Legionella pneumophila causes Legionnaires' disease. It is contracted by breathing in drops of contaminated water. Outbreaks are often reported in hotels, cruise ships, and office buildings, where people are exposed to contaminated droplets from cooling towers and evaporative condensers. They have also been reported in people who have been near whirlpools and saunas. Legionella pneumophila is not passed from person to person. Some experts believe the organism causes 29 - 47% of all pneumonia cases.

Legionnaires' disease was first described in 1976 after an outbreak of fatal pneumonia at an American Legion convention. The newly described organism that caused the disease was named Legionella pneumophila, shown in this picture. (Courtesy of the Centers for Disease Control and Prevention.)

Viral Pneumonia

A number of viruses can cause pneumonia, either directly or indirectly. They include:

• Influenza (Flu). Pneumonia is a major complication of the flu and can be very serious. Influenza-associated pneumonia is particularly risky for the elderly and people with heart disease. It can develop about 5 days after flu symptoms start. The flu weakens the body's defense systems, making it easier for bacteria to grow in the lungs.
• Respiratory syncytial virus (RSV). Most infants are infected with RSV at some point, but it is most often mild. However, RSV is a major cause of pneumonia in infants, as well as in adults with damaged immune systems. Studies indicate that RSV pneumonia may be more common in adults, especially the elderly, than previously thought.
• Severe acute respiratory syndrome (SARS). SARS is a respiratory infection caused by a coronavirus, which appears to have jumped from animals to humans. The disease was first reported in China in 2003.
• Human parainfluenza virus. This virus is a leading cause of pneumonia and bronchitis in children, the elderly, and patients with damaged immune systems.
• Adenoviruses. Adenoviruses are common and usually are not problematic, although they have been linked to about 10% of childhood pneumonias. Adenovirus 14 has been linked to an outbreak of severe community-acquired pneumonia in the Pacific northwest.
• Herpes viruses. In adults, herpes simplex virus and varicella zoster (the cause of chickenpox) can cause pneumonia in people with impaired immune systems.
• Avian influenza. Type A influenza subtype H5N1 in birds is spreading around the globe. Fortunately, only a few hundred human cases have been identified. Most have resulted from close contact with infected birds. The virus does not seem to spread easily from person to person. All patients diagnosed with "bird flu" show signs of pneumonia, although symptoms may be mild. Oseltamivir (Tamiflu) is the most effective treatment for this type of influenza, which can be fatal.

Aspiration Pneumonia and Anaerobic Bacteria

The mouth contains a mixture of bacteria that is normally harmless. However, if this mixture reaches the lungs, it can cause a serious condition called aspiration pneumonia. This may happen after a head injury or general anesthesia, or when a patient takes drugs or alcohol. In such cases, the gag reflex doesn't work as well as it should, so bacteria can enter the airways. Unlike other organisms that are inhaled, bacteria that cause aspiration pneumonia do not need oxygen to live. These bacteria are called anaerobic bacteria.

Opportunistic Pneumonia

Impaired immunity leaves patients vulnerable to serious, life-threatening pneumonias known as opportunistic pneumonias. They are caused by organisms that are harmless to people with healthy immune systems. Infecting organisms include:

• Pneumocystis carinii, renamed Pneumocystis jiroveci in 2002, is an atypical organism. Originally thought to be protozoa, it is now classified as a fungus. P. jiroveci is very common and generally harmless in people with healthy immune systems. It is the most common cause of pneumonia in AIDS patients.
• Fungi, such as Mycobacterium avium
• Viruses, such as cytomegalovirus (CMV)

In addition to AIDS, other conditions put patients at risk for opportunistic pneumonia. They include cancers, such as lymphoma and leukemia. Long-term use of corticosteroids and drugs known as immunosuppressants also increases the risk for these pneumonias.

Occupational and Regional Pneumonias

Exposure to chemicals can also cause inflammation and pneumonia. Where you work and live can put you at higher risk for exposure to pneumonia-causing organisms.

• Workers exposed to cattle, pigs, sheep, and horses are at risk for pneumonia caused by anthrax, brucella, and Coxiella burnetii (which causes Q fever).

• Agricultural and construction workers in the Southwest are at risk for coccidoidomycosis (Valley fever). The disease is caused by the spores of the fungus Coccidioides immitis.
• Those working in Ohio and the Mississippi Valley are at risk for histoplasmosis, a lung disease caused by the fungus Histoplasma capsulatum.

• Workers exposed to pigeons, parrots, parakeets, and turkeys are at risk for psittacosis, a lung disease caused by the bacteria Chlamydia psittaci.
• Hantavirus, a rare virus carried by rodents, causes a dangerous form of lung disease. It does not spread from person to person. Cases have occurred in New Mexico, Arizona, California, Washington, and Mexico.

Severe Acute Respiratory Syndrome (SARS)

Severe acute respiratory syndrome (SARS) is a contagious respiratory infection. The World Health Organization (WHO) officially identified SARS as a worldwide threat in 2003, and issued an unprecedented travel advisory. It wasn't clear at the time whether SARS would become a global pandemic or settle into a less aggressive pattern. The latter seems to have happened.

As of May 2005, there was no known SARS transmission anywhere in the world, according to the U.S. Centers for Disease Control and Prevention (CDC). The SARS outbreak is a dramatic example of how quickly world travel can spread a disease. According to reports from the CDC and WHO, more than 8,000 people became sick with SARS during the outbreak. Of that group, 774 died. The outbreak is also an example of how quickly a networked health monitoring system can respond to an emerging threat.

Causes And Risk Factors. SARS is a serious form of atypical pneumonia that causes acute respiratory distress and sometimes death. It is caused by a new member of the coronavirus family (the family that includes the virus that causes the common cold). The discovery of the SARS-related virus represents one of the fastest identifications of a new organism in history.

SARS is spread by droplet contact. When someone with SARS coughs or sneezes, infected droplets are sprayed into the air. Like other coronaviruses, the SARS virus may live on hands, tissues, and other surfaces for up to 6 hours in these droplets, and up to 3 hours after the droplets have dried.

While droplet transmission through close contact has been responsible for most cases of SARS, there is evidence that SARS might also spread by infected droplets carried on hands and other objects the droplets touch. Airborne transmission was a real possibility in some cases. The live virus was even found in the stool of people with SARS, where it has been shown to survive for up to 4 days. The virus may also be able to live for months or years when the temperature is below freezing.

The estimated incubation period is 2 - 10 days, although there have been documented cases where the start of illness was considerably faster or slower. People with active symptoms of illness are clearly contagious. It is not known, however, how early people begin to be contagious before symptoms appear, or how long they might be contagious after the symptoms have disappeared.

Prevention. The best way to prevent SARS is to avoid direct contact with people who have SARS until 10 days after their fever and other symptoms are gone. Reduce travel to locations where there is an uncontrolled SARS outbreak. The CDC has identified hand hygiene as the cornerstone of SARS prevention. Wash your hands often with soap and water, or use an alcohol-based instant hand sanitizer. Cover your mouth and nose when sneezing or coughing. Consider respiratory secretions infectious. Clean commonly touched surfaces with an Environmental Protection Agency (EPA)-approved disinfectant. In some situations, masks and goggles may help prevent the spread of airborne or droplet infection. Wear gloves when handling potentially infectious secretions.

Prognosis. The overall worldwide death rate from SARS at the end of the outbreaks was 14 - 15%, although it was up to 50% in infected people over age 65. Many more were sick enough to require breathing assistance from a machine (mechanical ventilation). Others needed to be treated in the intensive care unit (ICU).

Today, intensive public health policies are proving to be effective in controlling outbreaks. Many nations have stopped the epidemic within their own countries. All nations must be vigilant, however, to keep this disease under control.

Complications. Complications from pneumonia can include:

• Heart failure
• Liver failure
• Myelodysplastic syndromes (bone marrow abnormalities leading to anemia, low platelet counts, and low white blood cell counts)
• Respiratory failure