the largest is that which
produces pleuropneumonia in cattle, and this alone has been
cultivated. It gives a slight opacity to the culture fluids, and when
magnified two thousand diameters appears as a minute spiral or round
or stellate organism having a variety of forms. Its size is such that
it passes the coarse, but is held back by the finer, filters and it is
possible that this does not belong to the same class with the
others.[1] The diseases produced by the filterable viruses taken as a
class show much similarity. They run an acute course, are severe, and
the immunity produced by the attack endures for a long time.

Considered in its biological relations, infection is the adaptation of
an organism to the environment which the body of the host offers. It
is rather singular that variations in organisms represented by such
adaptation do not more frequently arise, in which case new diseases
would frequently occur. It cannot be denied that new diseases appear,
but there is no certain evidence that they do, and there is equally no
evidence that diseases disappear. From the meagre descriptions of
diseases, usually of the epidemic type, which have come down to us
from the past, it is difficult to recognize many of the diseases
described. The single diseases are recognized by comparing the causes,
the lesions and the symptoms with those of other diseases, and new
diseases are constantly being separated off from other diseases having
more or less common features. Many new diseases have been recognized
and named, but it is always more than probable that previously they
were confounded with other diseases. Smallpox is such a characteristic
disease that one would think it would have been recognized as an
entity from the beginning, but although the description of some of the
epidemics in remote times conform more or less to the disease as we
know it, the first accurate description is in the eighth century by
the Arabian physician Rhazes. Cerebro-spinal meningitis was not
recognized as a separate disease until 1803, diphtheria not until
1826, and the separation between typhoid and typhus fever was not made
before 1840. Nor is it sure that any diseases have disappeared,
although there seems to have been a change in the character of many.
It is difficult to reconcile leprosy as it appears now with the
universal horror felt towards it, due to the persistence of the old
traditions. It is possible, however, that the disease has not c