Meningitis is a serious illness in which the thin covers around the brain and spinal cord become swollen and angry. Dictionaries sometimes say 'the outer part of the brain swells', which is a fair picture for a beginner, though actually those covers (the meninges) are the part on fire — the brain itself can get into trouble too. It may come from a virus or from bacteria. The bacterial kind can move very fast and is a true emergency: a fierce headache, a stiff neck, fever, light that hurts the eyes, sometimes a rash. Viral meningitis is often less dangerous, but a doctor must sort it out; you cannot tell from the sofa. Some kinds can be stopped with a vaccine. In English it stays a medical word. People do not use it loosely for 'a really bad headache', and that is just as well, because missing the real thing can cost a life.