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 die.  Chance enters into this, but plays a small part, for the same varying individual susceptibility is shown experimentally.  If a given number of animals of the same species, age and weight, even those from the same litter, be inoculated with a given number of bacteria shown to be pathogenic for that species, the results differ.  If the dose be necessarily fatal, death will take place at intervals; if a dose smaller than the fatal be used, some animals will die, others will recover.  The defences of the organism being centred in the activity of the living tissue, any condition which depresses cell activity may have an effect in increasing susceptibility to infection.  Animals which ordinarily are not susceptible to infection with a certain organism may be made so by prolonged hunger, or fatigue, by the influence of narcotics, by reduction of the body temperature, by loss of blood.  In man prolonged fatigue, cold, the use of alcohol to excess and even psychic depression increases susceptibility.  It has been shown that such conditions are accompanied by a diminution in the power of the blood to destroy bacteria.

There is variation in the susceptibility to infection in the different races of man.  If a race be confined to one habitat with close intercourse between the people, such a race may acquire a high degree of immunity to local diseases by a gradual weeding out of the individuals who are most susceptible.  A degree of comparative harmony may be gradually established between host and parasite, as is the case in wild animals.  These have few diseases, the weak die, the resistant breed; they harbor, it is true, large numbers of parasites, but there is mutual adjustment between parasite and host.  Diseases in animals greatly increase under the artificial conditions of domestication.  Certain highly specialized breeds of cattle, as the Alderneys, are much more susceptible to tuberculosis than the less specialized.  The high development of the variation which consists in a marked ability to produce milk fat is probably combined with other qualities, shown in diminished resistance to disease, and under natural conditions the variation would not have persisted.  The introduction of a new disease into an isolated people has often been attended with dire consequences.  It is much the same thing with the introduction of disease of plants.  In Europe the brown-tail moth and the gypsy moth produce continuously a certain amount of damage to the trees, but their parasitic enemies have developed with them and check their increase.  These pests were brought to this country in which there were no conditions retarding their increase and have produced great damage.