Pneumonia can affect any person at any stage of life, even if they are otherwise healthy. Any factors that compromise the ability of the immune system to fight infection, or anything that adversely affects lung function, can significantly increase the risk of contracting pneumonia. Factors that could increase the risk of pneumonia include:

• Age. Pneumonia is more common and sometimes more severe in children and older adults. This is because the immune systems of children have often not matured enough to fight strong infections and the immune systems of older adults have often been weakened by age and other medical conditions. Also, children and older adults tend to spend more time in close contact with many other people and so are more likely to be in areas where bacteria and viruses are spread. For example, pneumonia can spread quickly in day care facilities and nursing homes.
  
  
• HIV/AIDS. HIV (human immunodeficiency virus) and AIDS (acquired immunodeficiency syndrome) are major risk factors for many infections, including pneumonia. When someone has HIV or AIDS, their immune system is seriously impaired. This means that many viruses or bacteria that are usually harmless for healthy people can result in serious illness for people with HIV or AIDS. People with these conditions are particularly at risk for pneumocystis jiroveci pneumonia (PCP) and are encouraged to take medication to prevent it. For more information, see HIV & AIDS.
  
  
• Other chronic diseases. Certain illnesses can also impair the immune system and increase the risk of pneumonia. These include:
  
  
• Cardiovascular disease
  
  
• Lung diseases such as emphysema
  
  
• Diabetes mellitus
  
  
• Sickle cell anemia

Additionally, people undergoing chemotherapy for cancer, as well as those who have had organs removed or transplanted, are at an increased risk for pneumonia. Other risk factors include:

• Extended stays in healthcare facilities. People who spend time in hospitals are at considerable risk for hospital-acquired pneumonia. This is especially the case for those who spend time in the intensive care unit or are aided in breathing by a mechanical respirator. Mechanical breathing devices may bypass many of the defense mechanisms that the body has to keep germs from entering the lungs. Also, strains of pneumonia contracted while hospitalized can be more dangerous than other types of pneumonia because of the increased likelihood that they have become resistant to certain antibiotics.
  
  
• Smoking. Smoking has a devastating effect on many parts of the body, especially the respiratory system. Smoking can damage the fine hair-like structures (cilia) that line the bronchial tubes and trap contaminants that enter through the mouth and nose. It can also damage the air sacs (alveoli) and the lining of the lungs. In addition, smoking suppresses the immune system, making it harder to fight off infections.
  
  
• Alcoholism or drug abuse. People who abuse alcohol or other drugs are at an increased risk of pneumonia for a number of reasons. Alcohol and drugs weaken the immune system, making it easier for germs to enter the respiratory system. Also, people who lose consciousness from excess alcohol or drugs are more likely to inhale vomit or other toxins from the stomach, which can lead to aspiration pneumonia.

The causes of pneumonia are varied.  Physicians often find that they may not be able to identify the cause even after extensive testing. Some causes of pneumonia include:

• Bacteria such as Streptococcus pneumoniae, Haemophilus influenzae or Legionella pneumophila (the bacteria that causes Legionnaire’s disease).
  
  
• Viruses such as influenza, measles or chickenpox.
  

  

• Fungi such as Histoplasma capsulatum or Coccidioides immitis.
  
  
• Parasites such as Toxoplasma gondii or Ascariasis.
  
  
• toxins from the stomach that have been inhaled into the respiratory system.
  
  
• Chemical toxins such as pesticides or gasoline.