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.