Complications of pneumonia are more common in older people, young children and people with existing health conditions, such as diabetes.

If you develop complications, you will be admitted to hospital for treatment.

The most common complications of pneumonia are pleurisy, a lung abscess and blood poisoning (septicaemia), described below.

Pleurisy

Pneumonia can cause pleurisy, which is inflammation of the pleura – the two thin linings between your lungs and your ribcage.

Less commonly, fluid can build up in the space between your lungs and the wall of your chest. This is called a pleural effusion.

Pleural effusion affects around half of people admitted to hospital with pneumonia.

The fluid can put pressure on the lungs, making it difficult to breathe. Pleural effusion usually gets better by itself when pneumonia is treated.

In around 1 in 10 cases of pneumonia treated in hospital, the fluids that make up a pleural effusion become infected by bacteria, which causes a build-up of pus. This is known as empyema.

The infected fluid is usually drained using a needle or a thin tube. The most serious cases may need surgery to remove the pus and repair any damage to the pleura and lungs.

Lung abscess

A lung abscess is a rare complication of pneumonia and mostly seen in people who have a serious, pre-existing illness, or those with a history of severe alcohol misuse.

A lung abscess is a pus-filled cavity that develops inside the tissue of the lungs. Coughing up unpleasant-smelling phlegm and swelling in your fingers and toes are signs of a lung abscess.

Lung abscesses can often be treated using antibiotics. This usually involves an initial course of intravenous antibiotics, followed by antibiotic tablets for four to six weeks.

Most people start to feel better within three to four days. It's important to finish your recommended course of antibiotics, even if you feel perfectly healthy, to prevent re-infection of your lungs.

Around 1 in 10 people with a lung abscess need surgery to drain the pus out of the abscess or remove the affected section of the lung.

Blood poisoning

Another rare and serious complication of pneumonia is blood poisoning, also known as septicaemia.

Symptoms of septicaemia include:

• high temperature (fever) of 38ºC (100.4ºF) or above
• fast heartbeat and breathing
• low blood pressure (hypotension), which will cause you to feel dizzy when you stand up
• a change in mental behaviour, such as confusion or disorientation
• reduced urine flow
• cold, clammy and pale skin
• loss of consciousness

Once your blood has become infected, it's possible for the infection to spread to other organs in your body, such as:

• the outer layers of your brain (meningitis)
• the lining of your abdomen (peritonitis)
• the inner layer of your heart (endocarditis)
• your joints (septic arthritis)

These types of infections are called metastatic infections and are usually serious, requiring treatment with high-dose intravenous antibiotics.