BALANCED PATHOGEN ISM: EPIDEMIOLOGY 109 
It might well happen that the introfkiction of such "balanced" 
strains into new fields would lead to temporary disaster, as for example, 
the highly fatal epidemic of measles when this virus first gained a 
foothold in the South Sea Islands. 
The disco^'ery of serologically distinct strains within a culturally or 
clinically identical type of organism, as has recently been found to 
occur in meningitis, typhoid, tetanus, bacillary dysentery and pneu- 
monia, opens up an entirely new field for the consideration of the 
epidemiologist, diagnostician and clinician. 
The immunological specificity of some of these strains is quite re- 
markable. Antibodies which react with the homologous strain may fail 
to induce a reaction with other strains. For example, an antipneumo- 
coccus serum specific for one of the immunological types is relatively 
ineffective against other types of the organism. Antimeningococcus 
serum which will neutralize the poison of one type of organism does not 
afford protection against other strains. Protective vaccination with 
one strain of dysentery bacillus may not prevent infection with a 
different serological strain of the dysentery bacillus. 
The origin of these serological strains or types is unknown. Their 
distribution is yet to be determined. Their existence complicates 
the national problem of unrestricted immigration with the obvious 
attendant possibility of introducing alien serological strains into an 
area of "balance" with endogenous strains, or into an area or group of 
individuals protected by vaccination with the local serological type. 
Explosive epidemics may conceivably follow such unrestrained move- 
ments of alien strains of bacteria, with, temporarily at least, high 
mortalities. 
Until such time as the catalogue of serological strains is complete, 
the mingling of nations, races, or even of groups, must be of concern 
to those charged with the maintenance of the public health. 
Theobald Smith^ has mentioned the diphtheria bacillus as an 
organism which possibly exliibits a tendency toward a parasitic 
existence. The toxin of the diphtheria bacillus is not a poison specific 
for man; many animals, as the horse and guinea-pig, are very sus- 
ceptible to it. Yet the diphtheria bacillus itself is almost obligately 
a human pathogen. The ever-increasing occurrence of avirulent, 
non-toxin-producing strains which are otherwise perfectly typical, and 
the frequent occurrence of individuals whose serum contains small 
amounts of natural antitoxin might be interpreted as an indication 
that strains of this organism are becoming gradually accustomed to 
a purely parasitic existence in the upper respiratory tract of man on 
the one hand, and that man has acquired some specific resistance to 
the microbe on the other hand. 
The tubercle bacillus (typus humanus) is an excellent example of 
an exquisitely balanced pathogenic microorganism. Its metabolism is 
not markedly different from that of the host and the typical disease 
1 Am. Med., 1904, 8, 711, 
