The introduction of infective agents into animals is by no means followed by the same results in all cases, and we are thus met with the fact of varying susceptibility amongst animals. We are able to recognize several kinds of variation in this respect. Animals by their inherited constitution present various degrees of susceptibility. Then it is well known that in many cases when an animal has passed through a single attack of an infective disease it is rendered more or less insusceptible of a second attack. Again, by certain methods of procedure an animal may be rendered insusceptible in various degrees. The terms immunity and immune are used as equivalent to insusceptibility and insusceptible.