Some species are entirely parasitical on other lichens (e.g.
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.
Filamentous diatoms may be mounted like ordinary seaweeds, and, as well as all parasitic algae, should whenever possible be allowed to remain attached to a portion of the alga on which they grow, some species being almost always found found parasitical on particular plants.
When we consider that tuberculosis, diphtheria, cholera, tetanus, typhoid fever, anthrax, malaria and a host of other contagious diseases have each been proved to be of parasitical origin, an idea may be conve y ed of the range of the subject.