Meningitis

The majority of meningitis infections are acquired by blood-borne spread. An individual may have another type of infection (of the lungs, throat, or tissues of the heart) caused by an organism which can also cause meningitis. The organism multiplies, finds its way into the blood stream, and is delivered in sufficient quantities to invade past the blood-brain barrier.

Direct spread occurs when an already resident infectious agent spreads from infected tissue next to or very near the meninges, for example from an ear or sinus infection. Patients who suffer from skull fractures provide openings to the sinuses, nasal passages, and middle ears. Organisms which frequently live in the human respiratory system can then pass through these openings to reach the meninges and cause infection. Similarly, patients who undergo surgical procedures or who have had foreign bodies surgically placed within their skulls (such as tubes to drain abnormal amounts of accumulated CSF) have an increased risk of the organisms causing meningitis being introduced to the meninges.

The least common method by which the organisms causing meningitis are transmitted, but one of the most interesting, is called intraneural spread. This involves an organism spreading along a nerve, and using that nerve as a kind of ladder into the skull where the organism can multiply and cause meningitis. Herpes simplex virus is known to use this type of spread, as is the rabies virus.