Diseases in animals greatly increase under the artificial conditions of domestication.
Certain highly specialized breeds of cattle, as the Alderneys, are much more susceptible to tuberculosis than the less specialized.

The high development of the variation which consists in a marked ability to produce milk fat is probably combined with other qualities, shown in diminished resistance to disease, and under natural conditions the variation would not have persisted.

The introduction of a new disease into an isolated people has often been attended with dire consequences.

It is much the same thing with the introduction of disease of plants.

In Europe the brown-tail moth and the gypsy moth produce continuously a certain amount of damage to the trees, but their parasitic enemies have developed with them and check their increase.