ossible 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. 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. Analogies to such conditions are
given in plants. In certain plants it has been shown that from unknown
causes there appears a tendency to the production of variations. A
very beautiful herbaceous peony known as "Bridesmaid" after having
grown for a number of years in single form, in one year wherever grown
suddenly became double. The peculiar thing with the lower unicellular
organisms is that the changes which so arise do not tend to become
permanent, the organism reverts to its usual character, the disease to
its sporadic type.

A very fatal form of poliomyelitis has for a number of years prevailed
in Sweden. In the United States there have been continually a number
of single cases of the disease, and it is not impossible that a more
pathogenic strain of the organism has developed in Sweden and has been
imported into this country, giving rise to the much greater extension
of the disease in a number of places.

The most cursory study of the infectious diseases shows that there is
great variation in the susceptibility of individuals. Even in the most
severe epidemics all are not equally affected, some escape the
infection, others have the disease lightly, others severely, some di