Encephalitis is a doctor’s word for swelling of the brain. The swelling is inflammation: the brain has become sore and puffed in the way a cut on your hand can become red and hot, but this is inside the head, so it is serious. Viruses are a common cause; other germs, or the body’s own defences going wrong, can cause it too. People may have a fever, a bad headache, confusion, or worse. You will hear it in hospitals and in the news when someone is very ill. It is not a casual word, and it does not mean an ordinary headache or feeling tired. Though actually, swelling in everyday English can sound like a bump you can see. Here the swelling is inside, in the brain itself.