Pasteur found that by gradually increasing the temperature they could be grown at one hundred and ten degrees.

When grown at this temperature they were no longer so virulent and produced in animals a mild non-fatal form of anthrax which protected the animal when inoculated with the virulent strain.

The well known variations in the character of disease, shown in differences in severity and ease of transmission, seen in different years and in different epidemics, may be due to many conditions, but probably variation in the infecting organisms is the most important.
The protozoa, like the bacteria, are unicellular organisms and contain a nucleus as do all cells.

They vary in size from forms seen with difficulty under the highest power of the microscope to forms readily seen with the unaided eye.

Their structure in general is more complex than is the structure of bacteria, and many show extreme differentiation of parts of the single cells, as a firm exterior surface or cuticle, an internal skeleton, organs of locomotion, mouth and digestive organs and organs of excretion.