Some two hundred years ago, after it became known that minute animal parasites were associated with certain diseases and were the cause of them, it rapidly came to be believed that all our ills were in some way caused by such parasites, known or unknown. Further study and investigation failed to reveal the intruders in many instances and so it began to be doubted whether after all they were responsible for much that had been laid at their doors. Then after it was discovered that minute plant parasites, bacteria, were responsible for many diseases they in turn began to be accused of being the cause of most of the ills that the flesh is heir to.
In later years we have come to adopt what seems to be a more reasonable view, for we can see and definitely prove that neither of these extreme views was correct but that there was much truth in each of them. To-day we recognize that certain diseases, such as typhoid, cholera, tuberculosis and many others, are caused by the presence of bacteria in the body, and it is just as definitely known that such maladies as malaria and sleeping sickness are caused by animal parasites.
Then there is a long list of other epidemic diseases, such as smallpox, measles and scarlet fever, the exact cause of which has not been determined. Many of these are believed to be due to micro-organisms of some kind, and if so they will almost certainly sooner or later be found. Curiously enough most of the diseases in this last class and many of those in the first are contagious, while all that are caused by animal parasites are, as far as is known, infectious but not contagious.
INFECTIOUS AND CONTAGIOUS DISEASES
It is important that we keep in mind this distinction. By contagious diseases are meant those that are transmitted by contact with the diseased person either directly, by touch, or indirectly by the use of the same articles, by the breath or effluvial emanations from the body or other sources. Small-pox, measles, influenza, etc., are examples of this group. By infectious diseases are meant those which are disseminated indirectly, that is, in a roundabout way by means of water or food or other substances taken into or introduced into the body in some way. Typhoid, malaria, and yellow fever, cholera and others are examples of this class. Thus it is evident that all of the contagious diseases may be infectious, but many of the infectious diseases are not as a rule contagious, although some of them may become so under favorable conditions.
Just one example will show the importance of knowing whether a disease is contagious or infectious. Until a few years ago it was believed that yellow fever was highly contagious and every precaution was taken to keep the disease from spreading by keeping the infected region in strict quarantine. This often meant much hardship and suffering and always a great financial loss. We now know that it is infectious only and not contagious, and that all this quarantine was unnecessary. The whole fight in controlling an outbreak of yellow fever or in preventing such an outbreak is now directed against the mosquito, the sole agent by which the disease can be transmitted from one person to another.
EFFECT OF THE PARASITE ON THE HOST
We have seen how a few parasites in or on an animal do not as a rule produce any appreciable ill effects. This is of course a most fortunate thing for us, for the parasitic germs are everywhere.
There is perhaps "more truth than poetry" in the following newspaper jingle:
"Sing a song of microbes,
Dainty little things,
Eyes and ears and horns and tails,
Claws and fangs and stings.
Microbes in the carpet,
Microbes in the wall,
Microbes in the vestibule,
Microbes in the hall.
Microbes on my money,
Microbes in my hair,
Microbes on my meat and bread,
Microbes everywhere.
Microbes in the butter,
Microbes in the cheese,
Microbes on the knives and forks,
Microbes in the breeze.
Friends are little microbes,
Enemies are big,
Life among the microbes is—
Nothing 'infra dig.'
Fussy little microbes,
Millions at a birth,
Make our flesh and blood and bones,
Keep us on the earth."