We know there is a great deal of variation in the virulence of the different pathogenic organisms, and the virulence can be artificially increased and diminished.

In epidemics of meningitis the virulence of the organisms is increased, as is shown by the greater mortality.

It is highly probable that such epidemics are due to changes which arise in the organisms from causes we do not know and which increase their capacity for harm.

It is possible that such a change would convert a carrier into a case of disease, the organism acquiring greater powers of invasion.

Such a strain of organisms arising in one place and producing an epidemic could be transported to another locality and exert the same action, or similar changes in the organisms could arise simultaneously in a number of places.