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If you've been diagnosed with pneumonia, it means you've got an infection in your lungs. Pneumonia is a serious illness, and you'll feel very sick. You'll have a fever and a cough. You may also feel like you can't catch your breath. You'll be weak and tired. And you probably won't feel like eating or drinking.

You're more likely to get pneumonia if you're older than 65 or if you smoke or drink a lot. You're also more likely to get it if you've already got another serious illness that has made your body and its natural defenses against infection weaker. Children who are younger than 2 years old are also at an increased risk of getting pneumonia.

Pneumonia can be dangerous, especially if you are older or already ill. If you think you have pneumonia, you should call your doctor right away. If you get treatment quickly, and if you rest and follow your doctor's advice, you are likely to make a full recovery. If you are older or have another health condition, you may not recover as quickly as someone who is younger or healthier.

• Pneumonia is serious and can sometimes kill. If you think you have it, see your doctor right away.
• Most people with pneumonia can be treated at home. You need to go to the hospital only if you are very sick.
• Antibiotic drugs are the main treatment for pneumonia. The sooner you take them, the better your chance of making a good recovery.
• It's important to be sure that you recover completely. Go to your doctor if any symptoms, such as fever and cough, don't seem to be going away.
• There are vaccines that may protect you against pneumonia. Your doctor will probably recommend getting these if you are at a high risk of getting pneumonia because you are older or in poor health. There is also a special vaccine for children. It should be given to all babies between 2 months old and 23 months old.

To learn more, see Antibiotics and Vaccines that can help protect you against pneumonia.

You have two lungs. They sit in your chest, inside your rib cage. One sits on each side of your heart. Your lungs are a little like two spongy, elastic bags that fill up with air as you breathe in. They empty as you breathe out.

A moist layer of tissue (called the pleura) covers your lungs.

Your lungs are divided into sections, or lobes. Your right lung has three lobes. Your left lung has two.

Keeping your lungs healthy is important. They supply oxygen from the air you breathe to every cell in your body. When you breathe out, they get rid of carbon dioxide, which is a waste product made by your body.

Here's how air travels into your lungs.

• Your windpipe (also called the trachea) is the air passage that leads from your throat into your chest.
• In your chest, it divides into two smaller airways (called bronchi). Each airway leads into a lung.
• Inside your lungs, the airways divide into even smaller airways (called bronchioles). At the end of each of these smaller airways are little air sacs (called alveoli). There are about 3 million of these tiny, grapelike sacs.¹

When you breathe in, air travels down your windpipe, through the airways, into your lungs and then into the smaller airways and the air sacs.

The air sacs in your lungs are connected to the network of blood vessels that surrounds your lungs. Oxygen from the air you breathe in passes through the thin walls of the air sacs and into these blood vessels.

Then, it is carried back to your heart and pumped around your body. At the same time, carbon dioxide (which is made as a byproduct of breathing) passes from your blood into the air sacs. The carbon dioxide leaves your body when you breathe out.

Your lungs have another important job. They filter the air you breathe. This keeps your body from being invaded by germs and other harmful things, such as pollution. Germs are all around us. They live in the air, soil and water, and sometimes in our noses and throats. If they get into your body, they can multiply. If they do this, you can get an infection.

Your lungs keep you from getting infections in several ways.

• The airways in your lungs are lined with tiny hairs. These are covered with a sticky substance that traps harmful germs and particles. These hairs wave back and forth. This pushes harmful things out of your lungs.²
• Further down in the lungs there are special cells that fight any infection that has made it past the first hurdle (the tiny hairs).
• Coughing is another way your lungs get rid of harmful things. Coughing happens without you thinking about it. It is your body's way of trying to clear things out of your airways that might irritate them. This type of automatic action is called a reflex action.

You get pneumonia when harmful germs get past your lungs' defense systems and start to multiply. This causes an infection. Often, these germs are already in your nose and throat. Sometimes they are in tiny drops in the air (for example, after someone sneezes), and you breathe them in. You can get an infection in just one of your lungs or in both of them.

If an infection gets into your lungs, your lungs become inflamed. This is one of the ways that your body tries to fight an infection. What happens is that special chemicals are released by your body's cells. And this makes more blood flow to the infected area. Along with the blood, infection-fighting parts of the immune system also travel through your body to the site of the infection.

Even though inflammation is one of the ways that your body responds to an infection or an injury, it can cause other problems in your body. In the case of pneumonia, it irritates your airways and makes your lungs heavy and stiff because of the extra blood. This is why you have trouble catching your breath. Also, fluid made by the inflamed tissue builds up in the tiny air sacs at the end of your airways. And this makes it difficult for oxygen to come into your body and for carbon dioxide to leave.

Meanwhile, your immune system (which protects your body against invading germs) responds to the infection that has gotten into your lungs. For example, cells in your blood (called white blood cells) are an important part of your body's defense system. If you have an infection, the white blood cells travel to that area, where they surround the invading germs and destroy them.

If you have a serious infection, such as pneumonia, a blood test may show that you have more white cells than normal in your blood. This is often a sign that your body is working to fight off the infection.³

In most cases of pneumonia, the germs that cause it are bacteria. But pneumonia can also be caused by viruses. In rare cases, pneumonia is caused by another type of germ called a fungus. Sometimes pneumonia is caused by two different germs (bacteria and a virus).

Different types of germs cause different types of symptoms. Some germs may make you more ill than others. In about one-third of cases, doctors can't tell what type of germ caused the pneumonia.

The most common form of pneumonia is pneumococcal pneumonia. Pneumococci are bacteria. About half of all cases of pneumonia are caused by pneumococci.⁴

To learn more, see Germs that cause pneumonia.

There are vaccines that can help keep you from getting pneumococcal pneumonia. A vaccine is usually given as a shot and keeps you from getting a specific disease. The vaccine may make it less likely that you'll get pneumococcal pneumonia, but it won't help if you already have it. The vaccine may also keep you from getting some of the more severe complications of pneumonia.

To learn more, see Vaccines that help protect you against pneumonia.

If you're healthy, you probably won't catch pneumonia from someone else. But people often have small amounts of some of the germs that cause pneumonia in their nose and throat. And these can be passed on when someone sneezes or coughs or has other close contact, such as hugging or kissing. If you are older or already have a serious illness, these germs may get into your lungs and cause pneumonia. Some types of pneumonia are contagious, such as pneumonia caused by the flu virus.

If you have pneumonia, there are some things you can do to keep from spreading it around. You should:

• Cover your mouth and nose when you cough or sneeze
• Throw out your used tissues. Don't leave them lying around
• Wash your hands regularly and well
• Avoid contact with anyone who has an increased risk of getting pneumonia. This could be someone whose immune system has been weakened by chemotherapy or HIV or AIDS, or an elderly person.

Some people are more likely to get pneumonia than others. Doctors call the things that increase your risk for a disease risk factors. Having a risk factor doesn't mean you'll get pneumonia for sure. It just means you're more likely to get it than someone who doesn't have the risk factor.

You should be especially aware of the dangers of pneumonia if you or someone in your family:

• Is over 65 years old
• Is under the age of 2 years old
• Is a heavy smoker (smoking damages your lungs, so you're more likely to get an infection)
• Is a heavy drinker
• Already has a lung disease, such as asthma or emphysema
• Has recently had another illness, such as the flu, which could have weakened their immune system
• Has a long-term (chronic) disease, such as diabetes, heart disease or sickle cell disease
• Has AIDS, or leukemia or another type of cancer
• Takes medicine for an illness such as AIDS or cancer (some medicines can make it harder for your body to fight off an infection, such as those given for chemotherapy)
• Has had an organ transplant.

All these things affect your body's natural ability to fight off infections. This is why they put you at a higher risk for pneumonia.

If you fall into any of these groups, you can take steps to protect yourself from pneumonia. You can take extra care and look after yourself. And you may also want to consider being vaccinated.

For more, see Vaccines that help protect you against pneumonia.

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.