Novemser 8, 1918] 
and more complete returns, the facts and infer- 
ences to be drawn from them will be of great 
moment. 
The disease is carried from place to place 
by persons, not things or by the general atmos- 
phere, as was once supposed. Its rapidity of 
spread is due to its great infectivity, short 
period of incubation, missed cases and absence 
of timely precautionary measures. It would 
appear that an epidemic does not easily start, 
but once the flame is well kindled a conflagra- 
tion oceurs which can not be stopped. The 
epidemics stop themselves. This they do 
either by the exhaustion of the susceptible ma- 
terial, by a reduction in the virulence of the 
causative agent, or both. 
The causative agent is believed to be the 
bacillus of Pfeiffer; the means of transfer; 
the air and objects recently contaminated by 
the buccal and nasal secretions of those who 
harbor the virus. It is a fundamental assump- 
tion that influenza is produced when, and only 
when, material from the mouth or nose of in- 
fected persons gets into the mouth or nose of 
someone who is susceptible. As is plainly rec- 
ognized in respect to intestinal infections, the 
hand probably plays an important part in the 
transmission of influenza. Coughing and 
sneezing help greatly to spread infection. 
CHANGES IN VIRULENCE 
Tt has long been known that interchanges of 
bacteria occur commonly from mouth to mouth 
under ordinary conditions of social inter- 
course. Most of the organisms are harmless 
under normal conditions of health. That their 
virulence is sometimes increased, sometimes 
reduced, according to circumstances, appears 
to be certain. But what the circumstances are 
which raise or lower the virulence is conjec- 
tural. The Pfeiffer bacillus is no stranger to 
America ; it was believed to be present in many 
healthy persons before the present pandemic. 
To account for the pandemic it has been sug- 
gested that something must have happened to 
increase its virulence or a new and more ac- 
tive strain has appeared, or the susceptibility 
of those attacked has become greater. 
The conditions which govern susceptibility 
SCIENCE 
455 
are not understood. Good general health, ab- 
sence of fatigue and avoidance of cold and 
hunger are standard methods of prevention 
for the individual. Vaccination against pneu- 
monia is practicable but such preventive treat- 
ment is in the experimental stage as respects 
influenza. 
The belief that immunity is conferred by 
an attack is partly confirmed by the observa- 
tion that in Europe and America a prepond- 
erance of persons who have suffered in the 
present pandemie are relatively young persons, 
few of whom could have experienced the dis- 
ease during the pandemic of 1889-90. 
The weather has always been supposed to 
exert an influence upon influenza—the very 
name is derived from the effect which extra- 
terrestrial conditions were supposed to exert 
upon it. But although there has been a great 
deal of study of this subtle matter, little is 
known concerning it. It seems probable that 
the weather this fall has aggravated the disease 
and contributed to the incidence of pneu- 
monia. 
The epidemics which occurred in the spring 
of 1918 were like those which are taking place 
now, except that the disease was milder and 
there was less pneumonia. Until recently the 
influenza reported from Europe was of this 
mild type. It seems to have been as infectious 
as it isnow. Reports coming from all parts of 
Europe indicate that the percentage of persons 
attacked was about the same at that at pres- 
ent. 
Something seems to have occurred during 
the summer greatly to increase the virulence 
of the disease. During July and August a 
number of vessels plying between Europe and 
America experienced intense outbreaks of in- 
fluenza, accompanied by very fatal pneumonia. 
That cases of the disease were being brought 
into the country in this manner was stated in 
the daily press and in official reports in 
July. 
COMPARISON WITH OTHER PANDEMICS 
It is interesting to compare the present pan- 
demic with others, but it is impossible to say 
