The meningococcus resides in the nose or throat of health carriers - people who do not themselves fall sick because they have a measure of natural or acquired immunity.

They can however spread it to others, and this tends classically to occur during the dry season, from December to February in much of the meningitis belt.

If the meningococcus falls upon a susceptible (non-immune) person, it can invade the tissues of the nose and throat. It then multiplies rapidly and spreads into the blood stream and up to the brain, where it has a special affinity for the meninges.

The normally clear fluid that surrounds the brain quickly becomes cloudy and more viscous, increasing the pressure within the skull. Meningococci within the bloodstream have their own toxic effect, causing shock (a potentially fatal drop in blood pressure) and bleeding into the skin, which gives rise to the characteristic rash.

The case fatality rate for untreated meningococcal meningitis approaches 50 per cent, and even those people who survive may have subsequent brain damage causing disabilities such as paralysis or deafness.

Even when it treated early and appropriately, meningitis is an extremely dangerous disease and the patient cannot always be saved - the case fatality rate tends to be between 5 per cent and 15 per cent.