Pneumonia is an inflammation of your lungs, usually caused by infection. Bacteria, viruses, fungi or parasites can cause pneumonia.

What is it?

Pneumonia is an inflammation of your lungs, usually caused by infection. Bacteria, viruses, fungi or parasites can cause pneumonia. Pneumonia is a particular concern if you're older than 65 or have a chronic illness or impaired immune system. It can also occur in young, healthy people.

Pneumonia can range in seriousness from mild to life-threatening. Pneumonia often is a complication of another condition, such as the flu. Antibiotics can treat most common forms of bacterial pneumonias, but antibiotic-resistant strains are a growing problem. The best approach is to try to prevent infection.


Pneumonia symptoms can vary greatly, depending on any underlying conditions you may have and the type of organism causing the infection. Pneumonia often mimics the flu, beginning with a cough and a fever, so you may not realize you have a more serious condition.

Common signs and symptoms of pneumonia may include:

  • Fever
  • Cough
  • Shortness of breath
  • Sweating
  • Shaking chills
  • Chest pain that fluctuates with breathing (pleurisy)
  • Headache
  • Muscle pain
  • Fatigue

Ironically, people in high-risk groups such as older adults and people with chronic illnesses or weakened immune systems may have fewer or milder symptoms than less vulnerable people do. And instead of having the high fever that often characterizes pneumonia, older adults may even have a lower than normal temperature.


Your body has ways to protect your lungs from infection. In fact, you're frequently exposed to bacteria and viruses that can cause pneumonia, but your body normally uses a number of defenses, such as cough and the normal microorganisms (flora) in your body, to prevent harmful organisms from invading and overwhelming your airways. However, numerous conditions, including malnutrition and systemic illness, can lower your protection and allow harmful organisms to get past your body's defenses and into your lungs.

Once the invading organisms are in your lungs, white blood cells — a key part of your immune system — begin to attack them. The accumulating invaders, white blood cells and immune system proteins cause the tiny air sacs in your lungs to become inflamed and filled with fluid, leading to the difficult breathing that characterizes many types of pneumonia.

Classifications of pneumonia

Pneumonia is sometimes classified according to the cause of pneumonia:

  • Community-acquired pneumonia. This refers to pneumonia you acquire in the course of your daily life — at school, work or the gym, for instance. The most common cause is the bacterium Streptococcus pneumoniae. Another, less common cause is Mycoplasma pneumoniae, a tiny organism that typically produces milder signs and symptoms than other types of pneumonia. Walking pneumonia, a term used to describe pneumonia that isn't severe enough to require bed rest, may result from Mycoplasma pneumoniae.
  • Hospital-acquired (nosocomial) pneumonia. If you're hospitalised, you're at a higher risk of pneumonia, especially if you're breathing with the help of a mechanical ventilator, in an intensive care unit or have a weakened immune system. This type of pneumonia can be extremely serious, especially for older adults, young children and people with chronic obstructive pulmonary diseases (COPD) or HIV/AIDS. Hospital-acquired pneumonia develops at least 48 hours after you're admitted to the hospital. This category includes post-operative pneumonia — most common in people older than age 70 who have abdominal or chest surgery — and health-care associated pneumonia — acquired in chronic care facilities, centers where drugs are given by intravenous drip (infusion) and kidney dialysis centers. A common predisposing factor for this type of pneumonia is gastro-oesophageal reflux disease (GORD). This occurs when some of the contents of your stomach flow back into the upper oesophagus. From there, the gastroesophageal contents can be inhaled (aspirated) into your windpipe and then into your lower airways. Even small amounts of gastroesophageal reflux can lead to pneumonia in people who are hospitalized.
  • Aspiration pneumonia. This type of pneumonia occurs when you aspirate foreign matter into your lungs — most often when the contents of your stomach enter your lungs after you vomit. This commonly happens when a brain injury or other condition affects your normal gag reflex. Another cause of aspiration pneumonia is consuming too much alcohol. Aspiration occurs when the inebriated person passes out and then vomits due to the effects of alcohol on the stomach. If someone's unconscious, it's possible to aspirate the liquid contents and possibly solid food from the stomach into the lungs, causing aspiration pneumonia. Difficulty swallowing, which occurs with diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease and strokes, may also lead to aspiration pneumonia.
  • Pneumonia caused by opportunistic organisms. This type of pneumonia strikes people with weakened immune systems. Organisms that aren't harmful for healthy people can be dangerous for people with AIDS and other conditions that impair the immune system, as well as people who have had an organ transplant. Medications that suppress your immune system, such as corticosteroids or chemotherapy, also can put you at risk of opportunistic pneumonia.
  • Other pathogens. Outbreaks of the H5N1 influenza (bird flu) virus and severe acute respiratory syndrome (SARS) have caused serious, sometimes deadly pneumonia infections, even in otherwise healthy people. Although rare, anthrax, plague and tularemia also may cause pneumonia. Some forms of fungi, when inhaled can cause pneumonia. Tuberculosis in the lung also can cause pneumonia. 

Risk factors

Factors associated with an increased risk of pneumonia include:

  • Age. If you're age 65 or older, particularly if you have other conditions that make you more prone to developing pneumonia, you're at increased risk of pneumonia. Very young children, whose immune systems aren't fully developed, also are at increased risk of pneumonia.
  • Certain diseases. These include immune deficiency diseases such as HIV/AIDS and chronic illnesses such as cardiovascular disease, emphysema and other lung diseases, and diabetes. You're also at increased risk if your immune system has been impaired by chemotherapy or long-term use of immunosuppressant drugs.
  • Smoking, alcohol abuse. Millions of microscopic hairs (cilia) cover the surface of the cells lining your bronchial tubes. The hairs beat in a wave-like fashion to clear your airways of normal secretions, but irritants such as tobacco smoke paralyze the cilia, causing secretions to accumulate. If these secretions contain bacteria, they can develop into pneumonia. Alcohol interferes with your normal gag reflex as well as with the action of the white blood cells that fight infection.
  • Hospitalization in an intensive care unit. Pneumonia acquired in the hospital tends to be more serious than other types of pneumonia. People who need mechanical ventilation are particularly at risk because the breathing tube bypasses the normal defenses of the upper respiratory tract, prevents coughing, may allow the stomach's contents to back up into the esophagus where they can be inhaled (aspirated), and can harbor bacteria and other harmful organisms.
  • Having COPD and using inhaled corticosteroids for more than 24 weeks. Research indicates that this greatly increases your risk of developing pneumonia, possibly serious pneumonia.
  • Exposure to certain chemicals or pollutants. Your risk of developing some uncommon types of pneumonia may be increased if you work in agriculture, in construction or around certain industrial chemicals or animals. Exposure to air pollution or toxic fumes can also contribute to lung inflammation, which makes it harder for the lungs to clear themselves.
  • Surgery or traumatic injury. People who've had surgery or who are immobilized from a traumatic injury have a higher risk of pneumonia because surgery or serious injuries may make coughing — which helps clear your lungs — more difficult, and lying flat can allow mucus to collect in your lungs, providing a breeding ground for bacteria.
  • Ethnicity. If you're a Native Alaskan or from certain Native American tribes, you're at greater risk for contracting pneumonia.


How serious pneumonia is for you usually depends on your overall health and the type and extent of pneumonia you have.

If you're young and healthy, your pneumonia often can be treated successfully. However, some organisms that cause pneumonia are so virulent that they overwhelm the defense mechanisms, even in otherwise healthy people.

If you have heart failure or lung ailments, especially if you smoke, or if you're older, your pneumonia may be harder to treat successfully. You're also more likely to develop complications, some of which can be life-threatening.

Pneumonia complications may include:

  • Bacteria in your bloodstream. Pneumonia can be life-threatening when inflammation from the disease fills the air sacs in your lungs and interferes with your ability to breathe. In some cases the infection may invade your bloodstream (bacteremia). It can then spread quickly to other organs.
  • Fluid accumulation and infection around your lungs. Sometimes fluid accumulates between the thin, transparent membrane (pleura) covering your lungs and the membrane that lines the inner surface of your chest wall — a condition known as pleural effusion. Normally, the pleurae are smooth, allowing your lungs to slide easily along your chest wall when you breathe in and out. But when the pleurae around your lungs become inflamed (pleurisy) — often as a result of pneumonia — fluid can accumulate and may become infected (empyema).
  • Lung abscess. A cavity containing pus (abscess) that forms within the area affected by pneumonia is another potential complication.
  • Acute respiratory distress syndrome (ARDS). The pneumonia involves most areas of both lungs, making breathing difficult and depriving your body of oxygen. Underlying lung disease of any kind, but especially COPD, makes you more susceptible to ARDS.


Your doctor may first suspect pneumonia based on your medical history and a physical exam. You may undergo some or all of these tests:

  • Physical exam. During the exam, your doctor listens to your lungs with a stethoscope to check for abnormal bubbling or crackling sounds (rales) and for rumblings (rhonchi) that signal the presence of thick liquid.
  • Chest X-rays. X-rays can confirm the presence of pneumonia and determine the extent and location of the infection.
  • Blood and mucus tests. You may have a blood test to measure your white cell count and look for the presence of viruses, bacteria or other organisms. Your doctor also may examine a sample of your mucus or your blood to help identify the particular microorganism that's causing your illness.