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More than 100 different germs can cause pneumonia.¹ Germs are often called micro-organisms. This means that they can't be seen without a microscope. Germs live everywhere: in the soil, air and water, and sometimes in your body. Pneumonia can be caused by bacteria, viruses or, more rarely, by a fungus.

This is the most common type of pneumonia. You can get it after you have had a cough or a cold. Several different types of bacteria can cause pneumonia. These bacteria live in the throats of many healthy people, but there aren't enough of them to cause an infection. However, if your body's immune system is weak because you are old or sick, these bacteria can invade your lungs and you can get pneumonia.

• Pneumococcal pneumonia. This is the most common type of bacterial pneumonia. It causes between one-fifth and three-fifths of all cases of pneumonia.² It's also known as streptococcal pneumonia.
• Legionnaires' disease. The bacteria that cause Legionnaires' disease usually grow in the water or ventilation systems of large public buildings. You are more likely to get this type of pneumonia if you smoke a lot or already have a lung disease.
• Aspiration pneumonia. You can get this type of pneumonia if you accidentally inhale phlegm, swimming-pool water or vomit. Doing this lets germs get into your lungs.³ This type of pneumonia is more common in people who have difficulty swallowing properly because they have another condition (if they have had a stroke, for example).

If you have pneumonia that's caused by bacteria, you will probably:

• Have a fever
• Have shaking and chills
• Have a cough. You might cough up phlegm that is green, rust-colored or yellow. Or you may have a dry cough where nothing comes up
• Have pain in your chest. This often hurts more when you take a deep breath
• Breathe faster than normal
• Have a faster pulse rate than usual
• Feel confused or disoriented.

See What are the symptoms of pneumonia? to find out more.

Viruses are another type of germ. Viruses often cause pneumonia in young children. The flu is also caused by a virus. So if you've just had the flu, you may get pneumonia, too.⁴ You can get bacterial pneumonia on top of the flu if your body has already been weakened by the flu.

If you have a flu vaccine, you may be less likely to get pneumonia. If you are protected against flu, it will be unable to weaken your immune system. A weak immune system can make you more vulnerable to pneumonia.

See Vaccines that help protect you against pneumonia to learn more.

If you have pneumonia that's caused by a virus, you will probably:

• Have a fever
• Have a dry cough
• Have a headache
• Have weak muscles
• Feel tired
• Feel out of breath.

See What are the symptoms of pneumonia? to find out more.

A type of germ called a fungus can also cause pneumonia. Fungal pneumonia is rare. It usually affects only people whose immune systems are weak. The symptoms of fungal pneumonia are similar to those of viral and bacterial pneumonia.

Another type of pneumonia is found in people whose defenses have been weakened by diseases such as cancer and AIDS. It's called Pneumocystis carinii pneumonia (PCP).

Mycoplasma pneumonia is a less severe form of pneumonia. Its main symptom is a cough that comes in bouts. People can feel sick to their stomach or vomit if they have this type of pneumonia. It spreads easily, and it often occurs in schools and day-care centers. Mycoplasmas are not really bacteria, and they're not viruses. They're sometimes called atypical bacteria. They can be treated with antibiotics.

More unusual forms of pneumonia include psittacosis (caused by a germ caught from contact with birds) and Q fever (caught from farm animals).

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.