Influenza, an epidemic disease, closely resembles, but is quite distinct from, cold in the head. It is characterised by early and marked debility and depression; though usually of short duration, attacks must not be disregarded; fatal results often ensue on carelessness. Convalescence is slow, and complications may ensue. The cause of the malady is obscure; sporadic cases always occur, but from time to time great epidemics of this disease have travelled westward over the world. Their movement seems to depend on atmospheric conditions, but is independent of the season of the year and often contrary to the direction of the wind. Visitations occurred in Britain in 1837-38, 1847-48, and 1889-91.