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, 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.
In up to half of cases, the brain abscess occurs as a complication of a nearby infection in the skull, such as:
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.
Infections spread through the blood are thought to account for around 1 in 4 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:
The most commonly reported infections and health conditions that may cause a brain abscess are:
Direct trauma to the skull can also lead to a brain abscess and is thought to be responsible for 1 in 10 cases.
The most commonly reported causes include:
In rare cases, a brain abscess can develop as a complication of neurosurgery.