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As to the structure of the bacterial cells but little can be said, except that these simple, unicellular plants consist of a transparent protoplasm enclosed in a cellular envelope or membrane. The

The very varied characters which distinguish different species of bacteria make it evident that there are essential differences in the living cell contents, or protoplasm, although these differences are not revealed by chemical analysis or by our optical appliances.

CHAPTER III

CHANNELS OF INFECTION

DISEASE germs gain access to the bodies of sus

ceptible individuals, giving rise to infectious diseases, through various channels. The most important of these are doubtless the alimentary and respiratory tracts, to which access is obtained through the mouth and nasal passages. It is sometimes difficult to ascertain whether infection has occurred as a result of the deposition of germs contained in inspired air upon the mucous membrane of the respiratory passages or by reason of their having been taken into the stomach with food or drink.

Germs suspended in the air would be to a considerable extent deposited upon the moist mucous membrane of the mouth and nasal passages, and would be carried thence to the stomach rather than to the lungs. However, in certain diseases, infection no doubt results from the deposition of germs in the bronchial tubes. This is true of pulmonary consumption, of

influenza (la grippe), of the pulmonic form of bubonic plague, of “wool-sorter's disease” (pulmonary anthrax), and of pneumonia. In diphtheria the initial infection commonly occurs upon the surface of the tonsils. This is also, no doubt, true in the various forms of tonsillitis and possibly in scarlet fever. Indeed there is good reason to believe that the tonsils constitute the avenue through which infection occurs, occasionally at least, in a considerable number of diseases of this class, including tuberculosis. The exact knowledge which has been gained during the past twenty years has made it evident that infection through the medium of the air is by no means as common as was formerly believed. We now know that malarial fevers and yellow fever are not contracted in this way, but that they result from inoculations made by infected mosquitoes. Some disease germs are quickly killed by desiccation and exposure to sunlight. These are not likely to be carried through the air in a living condition, and consequently the diseases produced by them are not propagated in this way. This is true of Asiatic cholera and to a considerable extent of typhoid fever, which diseases are recognised as being essentially water-born. However, the bacillus of typhoid fever resists desiccation for some time, and when the surface of the ground becomes contaminated with the discharges of typhoid

fever patients the bacillus may be carried by the wind, with dust, and deposited upon the moist mucous membrane of the mouths and nasal passages of individuals who breathe this dust-laden air. This is also true of the bacillus of bubonic plague, and to a still greater degree of the bacillus of tuberculosis.

The bubonic-plague bacillus, contained in the excreta of infected individuals and of rats, which are very susceptible to the disease, may retain its vitality for a considerable time when deposited upon the ground, and it is in this way that insanitary localities become centres of infection. The tubercle bacillus, which is contained in the matter coughed up from the lungs by persons suffering from pulmonary consumption, may retain its vitality and infecting power for a long time after the expectorated material containing it has been dried and pulverised.

It is evident that such dust is likely to be carried by currents of air and deposited in the lungs of persons who are compelled to live in localities where such insanitary conditions prevail. No doubt this is the usual way in which pulmonary consumption is contracted. Again, the dust deposited in the mouth and nasal passages may be swallowed and other forms of tubercular disease result — tubercular peritonitis, tubercular meningitis, tubercular joint disease, tuberculosis of the vertebral column.

In smallpox, scarlet fever, and the "eruptive fevers” generally infectious material is given off from the surface of the body of the sick person. This is associated with cast-off epithelium, pus cells, etc., and constitutes a kind of dust which abounds in the sickroom and clings to the clothing and bedding of the patient and of those in attendance upon him. In influenza and whooping-cough the patient forcibly ejects small masses of mucus which soon become desiccated and are likely to make up a portion of the dust in apartments occupied by such patients. Evidently the great danger from infection in these diseases results from visiting the sick-room or handling clothing which has been exposed to contamination by infectious material coming from the body of the sick person.

In typhoid fever, Asiatic cholera, and dysentery the infectious material coming from the sick person is contained in the discharges from the bowels and is usually quickly removed from the sick-room. The great danger as regards the spread of these diseases consists in the possibility that ignorant or careless persons may throw these discharges upon the ground or dispose of them in some way which makes it possible for the germs to be washed into a well or a running stream from which water is used for drinking purposes. But this is not the only way in which

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