Pneumonia is an inflammation of the lungs that is usually caused by infection. Pneumonia can also be caused by inhaling irritants such as vomit, liquids, or chemicals. With pneumonia, the air sacs in the lungs fill with liquid or pus, which interferes with the lungs' ability to transfer oxygen to the blood.

Before the invention of antibiotics in the 1930s, pneumonia was a leading cause of death. Though it has since become very treatable, pneumonia remains a public health problem.

There are many different kinds of pneumonia, ranging from mild to severe. There are 4 basic types:

• Community-acquired pneumonia (CAP), the most common type of pneumonia, is caused by bacteria, viruses, and other organisms that are acquired outside of the hospital or other health care settings.
• Hospital-acquired pneumonia (HAP) occurs at least 48 hours after someone has been admitted to the hospital. It can be caused by bacteria and other organisms that are usually different from CAP. HAP is usually more serious than CAP because the bacteria and organisms can be harder to treat, and because people who get HAP are already sick.
• Aspiration pneumonia occurs when liquids or other irritants are inhaled into the lungs. The most common type of aspiration pneumonia is caused by inhaling stomach contents after vomiting. People with medical problems (e.g., stroke, ALS) that affect swallowing are at an increased risk of this type of pneumonia.
• Opportunistic pneumonia occurs in people with weakened immune system (e.g., people with AIDS, cancer, organ transplant). Organisms that are not usually harmful to people with healthy immune systems cause these types of infections.

Pneumonia rarely causes serious complications for healthy people under 65 years of age. People who suffer from chronic respiratory diseases or who have compromised immune systems are generally at greater risk for developing pneumonia.

The most common causes of pneumonia are infections caused by:

• bacteria – the most common cause of pneumonia in adults
• viruses – often responsible for pneumonia in children
• mycoplasma – organisms that have characteristics of bacteria and viruses that cause milder infections
• opportunistic organisms – a threat to people with vulnerable immune systems (e.g., Pneumocystis jirovecii pneumonia [formerly known as Pneumocystis carinii] in people who have AIDS)

Most types of pneumonia are transmitted in the same way as influenza or the common cold - by people's hands and by tiny droplets from their mouths and noses. In fact, the same viruses that cause colds and the flu can cause pneumonia. If they infect the throat, sinuses, and upper respiratory tract, they cause a cold. If they reach the lungs, they cause pneumonia.

Bacteria that live permanently in many peoples' throats cause some of the more severe types of pneumonia. Normally, the immune system keeps them in check. If someone is weakened by a throat virus, these bacteria can trickle down into the respiratory tract. Bacterial pneumonia is most often caused by the bacteria Streptococcus pneumoniae (pneumococcus).

Respiratory syncytial virus (RSV) is the most common cause of pneumonia in infants and young children. It peaks sharply around December through March and usually isn't a life-threatening disease, though some individuals can be seriously affected. Viruses cause about half of all cases of pneumonia.

Mycoplasma is one of the most common causes of the illness called "walking pneumonia," so-called because people who have it are not confined to bed.

A fungus called Pneumocystis jiroveciiis usually seen only in people who have AIDS. This parasite is normally harmless, but in people with HIV it can cause an aggressive and often fatal pneumonia.

In addition to infectious diseases, people can get pneumonia from chemicals that enter the lungs and inflame them. Aspiration pneumonia is caused by accidentally inhaling food, vomit, or digestive acid into the lungs. The inhaled substance may become infected, or it may inflame the lungs and cause them to fill with liquid.

You have a higher risk of pneumonia if you:

• are under one year of age or over 65
• are a smoker
• have a cold or flu
• have a weak immune system due to cancer therapy, HIV infection, or other disease
• are undergoing surgery
• have a problem with alcohol use
• have a chronic illness such as heart disease or diabetes
• have a chronic lung disease, such as asthma or chronic obstructive pulmonary disease

Symptoms of pneumonia can vary depending on the cause of the pneumonia and the general health of the person who has pneumonia.

Pneumonia almost always causes a cough, which often produces sputum. However, the cough may be dry in children and the elderly. Red-brown, green, or yellow sputum may be a sign of bacterial infection. Thin, whitish sputum is a possible indicator of mycoplasma or viral pneumonia.

In bacterial pneumonia, all or part of the lungs slowly fill with liquid in a process called consolidation. Some bacterial lung infections develop over just a few hours. There's usually a high fever, sometimes going up to 40.9°C (105°F).

Other possible symptoms include:

• shortness of breath
• shivering
• chills
• headache
• delirium (confusion)
• severe bad breath
• muscle pain
• weakness
• chest pain, especially when breathing deeply
• blue lips and nail beds from lack of oxygen in the blood

Viral pneumonias are less associated with causing the lungs to fill with liquid as compared to bacterial pneumonias. They are usually milder than bacterial infections. An exception is the influenza virus, which can be very serious.

Typically, viral pneumonia causes these symptoms:

• dry cough
• minimal sputum
• headache
• muscle pain
• weakness and fatigue
• moderate fever, up to about 39°C (102°F)
• chills
• shortness of breath
• blue lips and nail beds

Although different causes of pneumonia may produce different symptoms, these symptoms may overlap considerably, so symptoms alone are not used to determine the type of pneumonia that you may have.

When diagnosing pneumonia, your doctor will perform a physical examination and check for fever and swollen glands. They will also listen to your lungs with a stethoscope. Most bacterial infections and some viruses can be detected in sputum or by blood tests. The buildup of liquid in the lungs can be observed in a chest X-ray or a CT scan.

Pneumonias caused by bacteria can be treated with antibiotics, as can those caused by mycoplasma. It is extremely important to take antibiotics exactly as prescribed by your doctor and for the full course of the medication, even if you are feeling better.

Your doctor will advise you on ways to ease the symptoms of viral pneumonia. For people at high risk of influenza, antiviral medications may be considered to lower the risk of developing pneumonia. Fungal infections can be treated with antifungal medications.

There are a number of steps you can take to help prevent pneumonia. Taking good care of your general health is important. Eat plenty of fruits and vegetables, stay physically fit, and get enough sleep. This will keep your immune system strong. Quitting smoking can also reduce the risk of developing pneumonia.

Getting vaccinated is also important. The two most common types (influenza virus and pneumococcus) can be prevented with vaccines. Yearly influenza vaccination is being promoted for everyone in Canada. A pneumococcal vaccine developed for children to protect against disease caused by Streptococcus pneumoniae is recommended as part of the primary series of immunizations for infants. Pneumococcal vaccines are also recommended for smokers, people over the age of 65, and people with certain health conditions that increase their risk of developing pneumonia. There are 2 types of pneumococcal vaccines available in Canada:

• Pneumococcal conjugate vaccines (PCV13, PCV15, PCV20)
• Pneumococcal polysaccharide vaccines (PPV23)

Different pneumococcal vaccines are recommended based on factors such as whether you have certain medical conditions and your pneumococcal vaccine history. Ask your health care provider for more information.

See your doctor about any cough that continues to get worse after 3 or 4 days. If you cough up blood or odd-coloured, foul-smelling sputum, you should see your doctor immediately. Even if the illness starts as a cold, bacteria can easily cause a secondary infection that could lead to serious pneumonia.

Good hand washing is an important way to limit the spread of the viruses and bacteria that can cause pneumonia. Wash your hands or use hand sanitizers frequently, especially after going to the bathroom and before you eat.