To understand what causes a chest infection, it is useful to learn about how the lungs work.
Your lungs are like two large sponges that are filled with tubes. As you breathe in, you take in oxygen through your nose and mouth. It then goes through a tube in your neck, called the windpipe or trachea.
The trachea splits into two tubes, one for each lung. These are called the primary bronchi. The bronchi divide into smaller and smaller bronchi which have tiny air sacs (alveoli) at the end. The oxygen is passed into your blood from the alveoli, before being pumped around your body by your heart.
As well as oxygen, bacteria and viruses in the air can also be passed down into your lungs. This usually does not cause problems because your immune system (the body's natural defence against infection) is able to kill the bacteria or viruses.
However, infection can occasionally take hold, particularly if your immune system has been weakened by other conditions, or your lungs have been irritated by cigarette smoke.
Most cases of acute bronchitis are caused by viruses, although sometimes they are caused by bacteria.
Acute bronchitis is a temporary inflammation of the trachea and the major bronchi, caused by infection. The inflammation causes swelling of the airways and a build-up of phlegm (thick mucus), which is cleared from the airways by coughing (which can sometimes last for up to three weeks).
Pneumonia is an infection of the tissues of the lung. Germs that cause infections, such as pneumonia, are often passed around in the community. In some people, the germs cause pneumonia to develop.
In adults, the most common cause of pneumonia is a type of bacterium called Streptococcus pneumoniae. This form of pneumonia is sometimes called pneumococcal pneumonia.
Less commonly, other types of bacteria can cause pneumonia, including:
- Haemophilus influenzae
- Staphylococcus aureus
Viruses can also cause pneumonia, most commonly the respiratory syncytial virus (RSV). Other viral causes include:
- varicella-zoster (the virus that causes chickenpox)
- the flu (influenza) type A or B virus
Viral pneumonia tends to be more common in young children than in adults.
Some groups of people have a higher risk of developing pneumonia, such as:
- babies and very young children
- elderly people
- people who smoke
- people with other health conditions
- people with a weakened immune system (the body's natural defence against infection)
Other health conditions that increase the risk of pneumonia developing can include:
- another lung condition, such as asthma or cystic fibrosis
- a heart condition
- a kidney or liver condition
- a lowered immune system
Your immune system can be lowered as a result of:
- a recent illness, such as flu
- treatment for cancer, such as chemotherapy
- taking medicines that suppress the immune system after an organ transplant
- a health condition, such as HIV or AIDS