20 
THIRTEENTH REPORT. 
fection. Then came the wholly unexpected observation that the blood 
of several goats agglutinated or clumped the micrococcus, thus in- 
dicating that the goats had in all probability the same disease. The 
Commission then examined thousands of the goats on the Island of 
Malta and found that fully 50 per cent gave the agglutination test while 
10 per cent were actually secreting the micrococcus in the milk. 
The surprising fact was thus established that the disease was really 
a milk infection and that the disease was primarily one of the goat. 
Equally astounding was the demonstration that the seemingly healthy 
goats continue to eliminate the specific germ in their milk for many 
months after the disease had passed away. The recognition that Malta 
fever was a milk borne disease and that the goat was probably the only 
carrier enabled the institution of the simplest measures for its sup- 
pression. 
CARRIERS. 
This brief statement covering the main facts in regard to Malta fever, 
serve to bring forward an exceedingly important factor in the trans- 
mission of communicable diseases, namely the existence of so-called 
“ carriers ” This term is applied to animals or persons who, though 
apparently in perfect health, harbor and eliminate a given disease germ. 
Usually, though not necessarily, the carrier has recovered from an at- 
tack of the disease and therefore enjoys immunity. This immunity 
while affording perfect protection against the injurious organisms is 
not always able to bring about the destruction of the latter which ac- 
cordingly persists in the body for a variable length of time. 
The existence of carriers is not altogether a matter of recent discovery, 
but rather a fact which has received wide confirmation and consequently 
has led to the recognition of its full importance. That apparently 
healthy persons could harbor in their mouths the germ of pneumonia, 
or of diphtheria became known in the early eighties about the time 
that these organisms were discovered. Later, it was shown that after 
recovery from diphtheria, the specific bacillus could persist in the throat 
in a small percentage of the cases for several weeks, and even months, 
and at the same time maintain its virulence. It was seen that such a 
carrier could spread the disease just as readily as the sick person and 
hence sanitarians very properly demanded that the isolation of the 
individual be continued after recovery and until the disappearance of 
the disease germ itself. 
It is only within the past few years that the role of carriers in typhoid 
fever has come to be fully realized. The majority of typhoid patients, 
after about the 12th day, eliminate the typhoid bacillus in the urine, 
at times in enormous numbers, and such elimination may continue for 
several weeks after complete recovery has taken place. Exceptionally 
the typhoid bacilli may persist in the urine or in the discharges for years, 
if not through the remainder of life. Hence efforts at the control of 
typhoid fever outbreaks must be directed quite as much against the 
possible carriers as against the actually sick. 
The studies on Malta fever, as pointed out, have shown that the goat 
is the natural carrier, the germ localizing in the mammary gland and 
hence appearing in the milk. In man this disease presents a condition 
analogous to that recognized in typhoid fever, that is to say, the germ 
