Acute bacterial meningitis is rapidly developing inflammation of the layers of tissue that cover the brain and spinal cord (meninges) and of the fluid-filled space between the meninges (subarachnoid space) when it is caused by bacteria.
Older children and adults develop a stiff neck that makes lowering the chin to the chest difficult or impossible, usually with a fever and headache.
Infants under 2 months old may not develop a stiff neck but may seem generally unwell and have a high or low body temperature, feed poorly, or be irritable or drowsy.
Bacterial meningitis is a medical emergency and is treated as soon as possible, before the diagnosis is confirmed.
To diagnose meningitis, doctors do a spinal tap (lumbar puncture) as soon as possible.