Animal diseases are not so commonly caused by fungi, unless we count the diseases of the human body produced by bacteria; such diseases as tuberculosis, diphtheria and indeed the majority of the infectious diseases of humanity. In these instances die spread of the disease may be said to be due always to dirt. Dust blowing about can carry infection directly or can introduce infection into some carrying agent, such as milk, that merely provides for distribution. Strict attention to cleanliness has been found to control the development of most bacteria and hence the spread of disease.
A radier spectacular method of disease control has, however, been placed in man’s hands through intensive research during recent years. This is the method of acquiring immunity. If a disease is given to an organism in a small quantity, well below a lethal dose, the disease may be killed through ‘antibodies ‘that it provokes to form in the body of the host. Immunity bodies can therefore be created in this way and collected for use in an emergency. The method has been tried in connection with plant diseases but with no conspicuous success. Indeed, it is hard to visualize how this method can be successfully used for plants because it depends on the presence of substances in animal blood.
The whole of the modern science of immunology and the relation of toxins and anti-toxins is extremely complicated. According to some investigators a substance called ‘bacteriophage ‘is formed that is actually a living organism which is even smaller than the bacteria, growing and preying upon them. A dramatic account of this discovery is given by Sinclair Lewis in his novel Martin Arrowsmith. Other scientists remain incredulous and prefer to explain the facts in terms of physical chemistry.
As, however, there are diseases called virus diseases, including those caused by vaccinia (small-pox) that seem to exist between the stage of chemical substances like enzymes and definite organisms like bacteria it can only be said that the whole subject is a developing one requiring further elucidation.
Unless the poisoning induced by poisonous toadstools can be called a disease it may be said that only certain microscopic fungi cause diseases of the animal body. There is a large group of fungi concerned in the formation of ringworm and allied diseases of the skin. These organisms have become so specialised in their host range that they are not found outside the diseased area which they characterise, but their nearest allies contain some that live normally as saprophytes on decaying fur and feathers. Presumably at some time there has been a rake’s progress from one stage to another.
Obscure intestinal diseases, such as the tropical ‘sprue ‘, are caused by fungi, and there are diseases, like ‘thrush,’ caused by a fungus growing on the throat-wall, and the well-known ‘pigeon-breeder’s disease ‘where a fungus grows in the human lung. From time to time other parts of the animal body are infected. There is a normal fungus-flora of the alimentary canal of man and occasionally associated organs, such as the liver, are attacked. Experiments to render animal bodies immune from fungus diseases have not so far been successful.
