Ancients infected by parasitical creatures evolved to become the Wraith.
If the disease is not treated, the parasitical infection can cause anemia, cardiac disease, kidney disease and endocrine failure.
A very remarkable instance of an acquired means of protecting a wound against parasitical invasion is to be found in granulations.
A plant or animal in perfect health is more resistant to parasitical invasion than one which is ill-nourished and weakly.
Each organism possesses within itself the means of protection against its parasitical enemies, and these properties are more in evidence when the organism is in perfect health than when it is debilitated.