This section is from the book "A Manual Of Pathology", by Guthrie McConnell. Also available from Amazon: A Manual Of Pathology.
A parasite is an organism that lives in or upon another. Many are harmless, but some of them are distinctly pathogenic, as they live at the expense of the individual, to the detriment of its well-being.
The body at whose expense the parasite lives is called the host.
Parasites may be divided into the vegetable and the animal varieties. Those living within the body are known as endo-parasites, those upon the surface of the body, as ectoparasites.
Parasitic diseases are characterized by having a specific exciting cause, and by the fact that they can be transferred from one individual to another. Some forms go through a portion only of their life history as parasites.
Others are able to live without the host, and are known as optional or occasional parasites. To this class belong many of the insects, as mosquitoes.
Some cannot live independently and are known as obligatory parasites, such as the tapeworms. They have no sense-organs, alimentary tract, or circulation. They have no need of such structures, as their food is taken up by absorption. The organs by which they retain their grasp and their powers of reproduction are well developed.
Pathologic conditions due to parasites may be the result of mechanical or chemical phenomena.
Mechanical, as obstruction of the lumen of an intestine, vessel, or duct; hemorrhage resulting from bites and suction, pressure.
Chemical: disturbances resulting from the absorption by the host of poisons, or from degenerative processes, causing reflex nervous symptoms, inflammation, and irritation.
Entamoeba coli. Entamoeba histolytica. Coccidium oviforme. Trichomonas intestinalis. Cercomonas intestinalis. Trichomonas vaginalis. Plasmodium malariae. Yellow fever. Trypanosomes. Pyrosoma.
Taenia cucumerina. Trematodes. Sucking worms.
Parogonimus westermanni. Nematodes. Round-worms.