The mortality is high, and even in recovery permanent damage is often done to the brain or to the organs of special sense.

Sporadic cases constantly occur in small numbers, and it is difficult or impossible to trace any connection between these cases.

At varying intervals, often twenty years intervening, an epidemic appears which sometimes remains local in a city or state, sometimes extends to adjoining cities or states, and may even extend over a very large area.

In the epidemics the mortality is much higher than in the sporadic cases.

The same explanation given for smallpox cannot apply here, for there is not a similar accumulation of susceptible material.