Meningitis occurs when inflammation from some type of infection affects the meninges, the tissue covering the brain and spinal cord. These infections may be bacterial, viral or fungal in origin. Rarely, meningitis may also occur in reaction to some medications or diseases (e.g., cancer). Anyone can develop meningitis, although it tends to occur most frequently in older adults, young children and individuals with a long-standing health condition (e.g., HIV/AIDS). However, cases of bacterial meningitis among adolescents and young adults may be becoming more common in the United States and United Kingdom.

The central nervous system includes the brain and spinal cord. Its neural tissues are protected by three membranous layers of connective tissue called meninges. The meninges also enable the vertebral column and spinal cord to flex and twist.

The outermost layer of the meninges is called the dura mater. It fuses with the lining of the skull. The middle layer of the meninges is called the arachnoid mater. This membrane is separated from the delicate, innermost membrane (pia mater) by a space that contains cerebrospinal fluid (CSF). This fluid carries nutrients, helps absorb impact and helps defend the brain from harmful microorganisms.

Bacteria, viruses or fungi reach the meninges through the bloodstream, from nearby infections (e.g., sinusitis) or by direct contact (e.g., penetrating injury, surgical procedure). While in the bloodstream, these microorganisms resist attack by white blood cells, causing receptors in the brain to allow penetration into the CSF. Once in the CSF, infection tends to progress rapidly due to the lack of disease-fighting substances (antibodies). The resulting inflammation causes oozing fluid called exudate to damage cranial nerves and pathways that carry signals to other parts of the body. It also causes swelling of the meninges, which disrupts the normal flow of blood and oxygen to the brain.