A brain abscess is usually caused by infection with either bacteria or fungi.

If the immune system is unable to kill an infection, it will try to limit its spread by using healthy tissue to form an abscess, to stop the pus infecting other tissue.

Infections of the brain are rare because the body has evolved a number of defences to protect this vital organ. One of these is the blood-brain barrier, which is a thick membrane that filters out impurities from blood before allowing it into your brain.

However, in some cases, germs can get through these defences and infect the brain.

Although the exact location of the original infection cannot always be identified, the most common sources are described below.

Infection in the skull

In up to half of cases, the brain abscess occurs as a complication of a nearby infection in the skull, such as:

• a persistent middle ear infection (otitis media)
• sinusitis (an infection of the sinuses, which are the air-filled cavities inside the cheekbones and forehead)
• mastoiditis (infection of the bone behind the eye)

This used to be a major cause of brain abscesses, but because of improved treatments for infections, a brain abscess is now a rare complication of these kinds of infection.

Infection through the bloodstream

Infections spread through the blood are thought to account for around one in four cases of brain abscesses.

People with a weakened immune system have a higher risk of developing a brain abscess from a blood-borne infection. This is because their immune system may not be capable of fighting off the initial infection.

You may have a weakened immune system if you:

• have a medical condition that weakens your immune system, such as HIV or AIDS
• receive medical treatment known to weaken the immune system, such as chemotherapy
• have an organ transplant and take immunosuppressant drugs to prevent your body rejecting the new organ

The most commonly reported infections and health conditions that may cause a brain abscess are:

• cyanotic heart disease – a type of congenital heart disease (a heart defect present at birth) where the heart is unable to carry enough oxygen around the body; this lack of a regular oxygen supply makes the body more vulnerable to infection
• pulmonary arteriovenous fistula – a rare condition in which abnormal connections develop between blood vessels inside the lungs; this can allow bacteria to get into the blood and, eventually, the brain
• dental infections or treatment for tooth decay
• lung infections, such as pneumonia or bronchiectasis
• infections of the heart – such as endocarditis
• skin infections
• infections of the abdomen – such as peritonitis (an infection of the bowel lining)
• pelvic infections – such as infection of the bladder lining (cystitis)

Infection after a head injury

Direct trauma to the skull can also lead to a brain abscess and is thought responsible for 1 in 10 cases.

The most commonly reported causes include:

• skull fracture caused by penetrating injury to the head 
• gunshot or shrapnel wound

In rare cases, a brain abscess can develop as a complication of neurosurgery.