Diseases which may be communicated from one person to another, or from an animal to a man, are known as infectious diseases. Some of these, such as itch, lice, ringworm, hydrophobia, and a few others, require actual contact with a diseased person or animal, and so are called contagious diseases. Some of the other infectious diseases, though actually transmitted in a different way, may also be conveyed by touch.
The commonest attacking the external parts, such as fleas, bugs, lice, and mosquitoes, are generally well known. They cause much irritation, with small lumps on the skin, and scratching leaves many marks on the body. The itch insect is very minute and microscopic, but as the female burrows under the skin and lays her eggs, small papules and pustules form, with very great irritation, and the body may be almost covered with an unsightly eruption. This disease can be communicated by touch to others. The head louse attacks the hair, and may be seen crawling about, or its eggs or "nits" can be seen fixed on to the hairs themselves. It causes much irritation, eruptions on the head, and lumps at the back of the neck.
Some insects are also of injury as conveyers of germ diseases. For instance, it is now known that the germs of malaria and yellow-fever are largely, if not solely, carried by mosquitoes. Flies also carry some diseases from one person to another, especially ophthalmia.
The animal parasites attacking the internal parts of the body are numerous. The commonest are tape-worms which get into the body with diseased meat of the cow or pig, and cause much irritation from their presence in the small intestine ; the common round worm, about twelve inches long, which also lives in the small intestine ; and thread or seat worms in the lower part of the large intestine, causing great discomfort. Very rarely in this country the trichina gets into the intestines and muscles of man from diseased pork. It is not easily killed or expelled if it has once got into the body. The other worms mentioned may be easily expelled by simple medicines, and any discomfort which they may have caused is thus removed. Another internal animal parasite, which is fortunately not very common, is the "bladder" form of the tape-worm of the dog. This bladder may begin to grow in some organ (generally the liver) of the human body, and cause great suffering, and even death, from its large size. It can only be removed effectually by a surgical operation.
These are all very minute, and only visible by the micro-scope, and their presence on or in the body is only judged from the diseases which they set up. They attack either the external or internal parts of the body. They may be all included under the one head of germs or micro-organisms. These are small, generally microscopic organisms of the lowest forms of vegetable life.
Germs may be carried from one person to another, and received by that person in different ways. They may be conveyed by actual contact, as in the case of ringworm, erysipelas, ophthalmia (infectious inflammation of the eyes), hydrophobia, small-pox, etc. The germs may possibly be taken in through the unbroken skin, but much more frequently through a small crack or sore in the skin. Secondly, they may be conveyed by the air, and taken in by the breath. This is by far the commonest method, as seen in whooping-cough, scarlatina, small-pox, diphtheria, measles, consumption, etc. Thirdly, they may be carried by water, and so taken into the stomach and intestines, as with cholera, typhoid fever, dysentery, etc. Fourthly, by the food, and taken to the stomach and intestines as before, as with typhoid fever, consumption, and foot-and-mouth disease (conveyed by milk). Fifthly, they may be carried by clothes, and so get into the air, as with scarlatina. They may also be carried by insects, such as flies and mosquitoes, as above stated. In some instances the method of conveyance is mysterious, as in the widely-prevalent influenza, whose history has so far baffled research.