ntists. Very early it was
believed that it might be transmitted through the air, and the fact that
infection usually occurred in the vicinity of the water and in the
tropics or in midsummer led to the belief that the disease was due to
fermentation. This theory received strong support in the fact that
serious outbreaks of the fever often followed the coming into port of
vessels from the tropics with the water in their holds in an offensive
condition. When it was discovered that bacteria were the cause of
fermentation and also of many diseases this theory was considered
abundantly proven. From time to time, announcements have been made that
the particular species of bacteria that causes the disease has been
isolated, but there has always been something lacking in the final
proof.

Yellow fever has always been regarded as a very highly contagious as
well as infectious disease, and the utmost precaution has been taken to
isolate the patients when possible and in recent years strict
quarantines have been established against infected localities and no
person or commerce or even the mails were allowed to come from such
places without thorough fumigations. But all these things proved
unsatisfactory. The disease could not ordinarily be checked by simply
isolating the patients. Many people became sick without ever having been
near a yellow fever patient, while others worked in direct daily contact
with the disease and did not suffer from it. Those who had once had it
and recovered became practically immune, rarely suffering from a second
attack. Negroes may suffer from the disease, but are usually regarded as
practically immune.

[Illustration: FIG. 104--Yellow-fever mosquito (_Stegomyia calopus_).
(R. Newstead, del.)]

It was early observed, too, that the danger zone might be quite well
defined and that outside this zone one would be safe. More than a
century ago the British troops and other inhabitants of Jamaica found
that by retreating to the mountains during the warm weather the
non-immunes could escape the fever. It was also observed that those who
slept on the first floor were more apt to take the disease than those on
the second floor.

THE YELLOW FEVER COMMISSION

In 1900, during the American occupation of Cuba, yellow fever became
very prevalent there. A board of medical officers was ordered to meet in
Havana for the purpose of studying the disease under the favorable
opportunities thus afforded. This board, w