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A brain abscess is usually caused when bacteria such as staphylococci and streptococci infect part of the brain. In response to the infection, there is inflammation and death of some of the tissues in the brain. Fluid, destroyed tissue cells, white blood cells, and live and dead microorganisms collect and form a mass. This mass usually becomes enclosed by a membrane that forms around it.

The brain swells in response to the inflammation, and the mass may put pressure on the the brain. Infected material can block the blood vessels of the brain, further damaging brain tissues. The pressure within the brain rises, causing more damage and problems.

A brain abscess may result from other infections, such as an epidural abscess, chronic ear infection, chronic sinusitis, or mastoiditis. The infection may be caused by direct spread of the microorganisms through the bones of the skull and through the membranes covering the brain (meninges).

The infection may also spread through the blood vessels from distant locations in the body. Lung infections such as empyema or lung abscess are common sources. Other sources include infections of the teeth, skin, bone, or heart. Direct head injury may also introduce microorganisms into the brain, and the infection may occasionally occur after surgery on the brain or spinal cord. In about 10%-25% of cases, no original source of the infection can be identified.

Symptoms may develop gradually or suddenly. There is usually little or no sign of general infection throughout the body. Early symptoms are usually headache (about 50% of cases), muscle weakness, loss of sensation, or seizures.

People at higher risk include those with congenital heart diseases, such as Tetralogy of Fallot, and congenital blood vessel abnormalities of the lungs, such as Osler-Weber-Rendu disease. These disorders carry a high risk of infection of the heart or lungs, which can then spread to the brain. Other risks include drug abuse, chronic ear infections, chronic sinusitis, infections that spread bacteria to the bloodstream (bacteremia), and any condition that harms the immune system.