CHAPTER XLIII.

INFECTIVE DISEASES.

The prevention of disease depends largely on a knowledge of its causes. Disease may be due to a personal life not in accordance with physiological laws; or to some cause or causes acting _ab extra_. With advance of knowledge the number of diseases which can be proved to be caused by a _contagium vivum_ introduced from without is steadily increasing. We have already discussed the influence of habits, of clothing, exercise, sleep, and food on health, and have shown how errors in these respects may lead to disease. It now remains to consider more particularly the prevention of diseases, due to the introduction into the system of contagia.

In the study of such diseases three chief factors require consideration: (1) the contagium itself; (2) conditions of environment, as climate, soil, season, weather, etc., which may favour or impede its spread; and (3) personal conditions which similarly influence it. Of these age, heredity, fatigue, injury, diet, and race are specially important.

The first two groups of diseases given in the Registrar-General’s classification of causes of death are (1) Specific Febrile or Zymotic Diseases, and (2) Parasitic Diseases. The objection to the word “specific” is that, although in most instances diseases in this group are “specific” in the sense that they are caused by a particular microbe, _e.g._ tetanus, anthrax, tuberculosis, in a few instances the same lesions may be caused by several microbes, _e.g._ septicaemia (blood-poisoning), pneumonia. “Zymotic” was the name given by Farr, in view of the analogy of the febrile process to that of alcoholic fermentation. In both there is the introduction of a living germ or germs; in both a period of “incubation” in which nothing can be observed; then follows the active disturbance; and in the disease, as well as in the fermenting liquid, the process is stopped, when the microbes have multiplied to a certain extent, a temporary or permanent protection being the result. The best name for the diseases in this group is “Infective.” Parasitic diseases, like ringworm, scabies, or trichinosis, are also infective; but for convenience may be described separately as “parasitic.”

The relation between the words “infectious” and “contagious” requires explanation. A disease like measles or small-pox, which can be transmitted from person to person, without immediate contact between the two, is termed =infectious=. In these cases the infection is conveyed by mucus expectorated or by dust blown about, or carried in apparel, etc., from the first patient. Such diseases may also, of course, be communicated by direct contact. If direct contact between the sick and well is indispensable for the transmission of a disease it is called =contagious=. There is no such hard line in nature, although some diseases can be more easily communicated than others. The term contagious is usually applied to parasitic diseases like ringworm and scabies, but even these can be communicated by means of infected articles as well as persons. The word contagious should be abandoned for all the acute febrile diseases. The word =infective= is used to include all specific febrile diseases, however spread. This word, therefore, includes not only infectious and contagious diseases, but also diseases spread by =inoculation=, _i.e._ injection of the infection under the skin. Thus malaria is not infectious from patient to patient; but can be inoculated by the mosquito.

=Infective Diseases= are either acute or chronic. Of _acute_ infective diseases small-pox and enteric fever are typical examples; of _chronic_, tuberculosis and syphilis.

It was formerly supposed that in certain diseases the contagium or infective agent grew in external noxious matter, a _miasm_ being produced; while in other diseases _contagion_ was only produced direct from patient to patient; and others originated in either way. Hence the classification of infective diseases into (_a_) miasmatic, (_b_) contagious, and (_c_) miasmatico-contagious diseases. This classification has now been abandoned. Thus influenza and ague were formerly thought to be miasmatic; but the former is spread by personal infection; the latter by inoculation of the contagium by an infected mosquito.

=Bacteriology= has thrown an immense light on the causation of infective diseases. A large number of these have been proved to be caused by bacteria, and by analogy we infer the same thing for many others. =Koch= has laid down the following =postulates= as necessary before it can be stated that a particular disease is directly caused by a given microbe:—

(1) The microbe shall be demonstrated in the diseased tissues or blood of man or an animal suffering or dead from the disease.

(2) The microbes shall be isolated from these and cultivated in suitable media until obtained in pure culture. That is to say, matter containing the microbe, taken from the infected source, must be cultivated in artificial media outside the animal body, under conditions excluding the possibility of the introduction of other microbes, until pure cultures of these microbes are obtained, and these microbes must be transplanted from generation to generation, until it is certain that no trace of non-living matter derived from the original animal body remains in the culture.

(3) A pure culture of the microbe, thus obtained, shall, when introduced into the body of a healthy susceptible animal, reproduce the disease in question.

(4) The microbe in question shall be found in the animal so affected. Kanthack adds a further condition, that

(5) The toxins and poisonous substances obtained from the artificial cultivations shall agree chemically and physiologically with those obtained from the diseased animal.

All the preceding conditions have been fulfilled for anthrax, diphtheria, and tetanus; and the first four conditions have been fulfilled in regard to tuberculosis, glanders, gonorrhœa, malignant œdema, and actinomycosis. In enteric fever and influenza the first two conditions have been met; but inoculation experiments (3) have failed. In leprosy and relapsing fever the first condition is met, but (2) has failed.

In the following diseases the specific microbe has not been isolated, though from analogy it is believed that each of them is caused by such a microbe:—

Measles. Rubella (German measles). Typhus fever. Scarlet fever. Varicella (chicken pox). Variola (small-pox). Whooping Cough. Mumps. Hydrophobia, etc.

Erysipelas occupies a special position. It is a specific disease due to a microbe, which, when it attacks other parts than the skin, may produce abscesses, boils, or blood-poisoning.

Bacteria are either =saprophytes=, _i.e._ they can grow on dead organic or even inorganic matter; or =parasites=, _i.e._ they are dependent for their existence on a living plant or animal which they invade. There are two varieties of each of these, obligate and facultative. An _obligate parasite_ can develop only within a living host; while a _facultative parasite_ can, according to circumstances, lead either a parasitic or saprophytic form of existence. The fact that certain contagia are completely, and others only partially, parasitic brings out important differences in their life-history. Thus, so far as we know, the contagia of scarlet fever, measles, small-pox and hydrophobia do not multiply outside the body. Hence there is a reasonable prospect of annihilating them by measures of disinfection and isolation. The position of diphtheria is doubtful. It may have a saprophytic phase of life. The contagium of tuberculosis, as well as of erysipelas, may have a life outside the host, though to what extent is doubtful. Cholera and enteric fever, although generally communicated by infection, appear sometimes to be communicated by contagia grown in saprophytic life, remote from preceding cases.

The infection caused by bacteria may be _local_ or _general_. Thus in tetanus and in diphtheria the invading bacteria usually remain at their original point of invasion (under the skin in tetanus, in the throat in diphtheria). In anthrax always, and often in enteric fever, they are present in the general circulation. In both instances the symptoms of disease are due chiefly to the toxic products or =toxins= formed by the bacteria. These toxins are enzymes, ptomaines, tox-albumins, etc. The specific toxins of anthrax, diphtheria, and tetanus have been identified; and by this means the possibility of neutralising them is created.

The =channels of infection=, _i.e._ of invasion of contagia, are the skin and the mucous membranes, particularly of the digestive and respiratory tracts.

The =Incubation Period= of an infectious disease is the interval elapsing between the receipt of infection and the earliest development of symptoms. The _period of incubation_ of the chief infectious diseases is shown in the following table:—

┌──────────────────┬─────────────────┬────────────────────────┐ │ DISEASE. │ BEGINS USUALLY │ BUT MAY POSSIBLY BE AT │ │ │ ON THE │ ANY PERIOD BETWEEN │ ├──────────────────┼─────────────────┼────────────────────────┤ │_Scarlet fever_ │ 4th day. │ 1 and 7 days. │ │ │ │ │ │_Diphtheria_ │ 2nd „ │ 2 „ 5 „ │ │ │ │ │ │_Small-pox_ │ 12th „ │ 1 „ 14 „ │ │ │ │ │ │_Chicken pox_ │ 14th „ │ 10 „ 18 „ │ │ │ │ │ │_Typhus fever_ │ 12th „ │ 1 „ 21 „ │ │ │ │ │ │_Enteric fever_ │ 14th-21st „ │ 1 „ 28 „ │ │ │ │ │ │_Cholera_ │ 1st-3rd „ │A few hours and 10 days.│ │ │ │ │ │_Measles_ │ 12th-14th „ │ 10 and 14 days. │ │ │ │ │ │_Rötheln_ │ │ │ │(_German measles_)│ 14th „ │ 12 „ 18 „ │ │ │ │ │ │_Mumps_ │ 19th „ │ 16 „ 24 „ │ │ │ │ │ │_Whooping cough_ │ 14th „ │ 7 „ 14 „ │ │ │ │ │ │_Influenza_ │ 2nd „ │ 2 „ 6 „ │ └──────────────────┴─────────────────┴────────────────────────┘

The period of incubation is several weeks in hydrophobia and syphilis, and may be several years in leprosy.

Following the period of incubation, come the premonitory symptoms, which usually are somewhat sudden in onset. For the chief =symptoms of onset= see page 318.

Persons vary in susceptibility to attack by different infective diseases. The intensity of an attack depends on the condition of the patient, and on the number and the virulence of the particular microbes infecting the patient. In certain families attacks of particular diseases are more severe, and attacks are more liable to occur than in others.

It has been shown in certain diseases that the cells and the fluids of the body have a protective effect against infection. This protective action varies in different persons, and in the same person at different times. The cells of the body (_phagocytes_) swallow up and destroy a certain number of bacteria. This action is called =phagocytosis=. It is overcome when the dose of contagium is excessive, or when the vitality of the individual is lowered, especially the local vitality at the part attacked. Thus children with “weak throats” are particularly prone to scarlet fever and diphtheria.

The =protection= afforded by =one attack= of an infective disease against its recurrence varies greatly. A second attack of small-pox is very rare, of scarlet fever less uncommon, of diphtheria common. In erysipelas, influenza, pneumonia, and rheumatic fever, second or even more numerous attacks are common.

=Immunity= against an infective disease may be _natural_, but is more often _acquired_ by an attack of the disease in question. This latter _immunity_ is _active_, and is due to the formation in the tissues of the immunised person or animal of substances produced by the reaction of these tissues to the stimulus of the contagium. Thus a pig when it has recovered from an attack of swine-plague has produced what are called in German _antikörpers_, and its tissues are now a medium unfavourable to the growth of the bacillus of swine-plague. If the serum of the protected pig is injected under the skin of another pig, the latter acquires _passive immunity_ against swine-plague, which is not so persistent as active immunity.

=Active Immunity= can be produced (1) by an attack of an infective disease, or (2) by artificial inoculation (under the skin) of the contagium of the disease, producing a milder attack of the disease. This may be done (_a_) by inoculating _small doses of a virulent contagium_, as in the inoculation of small-pox from a previous patient; or (_b_) by inoculating an _attenuated virus_, as in vaccination. Inoculation of small-pox virus usually produced a milder attack than infection by ordinary means; but patients thus inoculated were a great source of danger to other persons. In vaccination the virus of small-pox is employed, which has become attenuated by passing through the calf. In its passage, it has lost the power of producing anything beyond a vesicle at the point of inoculation. The principle of protecting by attenuated virus was extended by Pasteur, who was able to render animals resistant against anthrax, swine-fever, and quarter-evil, and hens against fowl-cholera, by inoculating them with attenuated cultures of the contagia of these diseases. Haffkine has applied the same method on a large scale for cholera.

The above are methods of bacterial vaccination. Salmon and Smith have shown that artificial active immunity can be produced also by (3) _toxin-injection_. They artificially cultivated the hog-cholera bacillus in broth. This broth was then sterilized, the bacilli being killed, but their products remaining. By injecting pigeons with this sterilized broth they made them resistant to subsequent infection by the bacillus itself, thus proving that immunity can be produced by chemical as well as by biological means. The immunity was proportional to the dose of the toxin absorbed. By gradually increasing the dose, it was found practicable to confer immunity, not only against doses of toxin that would otherwise have been fatal, but also against bacterial infection by the particular bacillus used in manufacturing the toxin.

=Passive Immunity.=—Behring and Kitasato found that if the toxin (free from the bacilli) of tetanus be injected into an animal in increasing doses until it becomes immune against infection by the bacilli of tetanus, the blood serum of the animal in question injected into white mice confers the same immunity on them. The protection thus conferred is only temporary. Exactly the same procedure has been adopted for diphtheria, and it is now found that by injecting _anti-diphtheritic serum_ into children who are exposed to the infection of diphtheria, they can for several weeks be prevented from developing the disease. This is of great practical importance, as meanwhile the source of infection can have been removed. Furthermore, the _protective serum is also curative_, and by its means diphtheria, if early treated, can be reduced from a dangerous to an insignificant disease.

Various =theories= have been propounded =to explain immunity=. Pasteur supposed that the special pabulum or food of the bacillus of the given disease became exhausted; but this does not fit in with the immunity that can be produced by toxins and anti-toxins. Chauveau supposed that certain bacterial products are retained in the body, rendering it unsuitable for further growth of the particular bacillus; just as more than 14 per cent. of alcohol in a saccharine solution prevents further fermentation. This does not explain all the facts. Metschnikoff concluded that the fight of the leucocytes and phagocytes of the body against weaker bacilli, gave the power of fighting and overcoming a more virulent bacilli, and that these properties of the cells were transmitted to later generations of body cells. This theory fails to explain the acquired immunity against toxins as well as against bacteria which occurs. The discovery that the blood and other normal tissue fluids possess some power of destroying bacilli has relegated the phagocytal theory to a secondary position.

=Natural Immunity= varies in different animals. Thus enteric fever, scarlet fever, and measles are not known to occur except in man. Tuberculosis, anthrax, hydrophobia (called rabies in the dog), glanders and tetanus are common to man and certain other animals. Man, cattle and pigs frequently suffer from tuberculosis; goats, sheep, horses, and dogs are practically immune to it.

=Epidemic and Endemic Diseases.=—Infective diseases may occur _sporadically_, in _epidemics_, or in _pandemics_, _i.e._ epidemics spread over a number of countries. The word =epidemic= is used here to mean specially prevalent, and not to apply only to infective diseases. Thus there may be an epidemic of arsenical poisoning from contaminated beer.

Certain infective diseases are =endemic= or topical, _i.e._ they have special homes or centres, from which they occasionally spread as epidemics. Yellow fever, cholera, and malaria belong to this group. In a minor degree enteric fever, epidemic diarrhœa, and tuberculosis may be described as endemic.

Each infective disease has a special =seasonal incidence=. Of these the most important are the _autumnal_ group, viz.

Epidemic Diarrhœa, maximum prevalence in July and August. Enteric Fever „ „ „ November, but excessive, Aug. to Dec. Erysipelas „ „ „ Nov. to Dec. Diphtheria „ „ „ Nov. and Dec., excessive, Sept. to Dec. Scarlet Fever „ „ „ Oct., excessive in Aug. to Dec.

Of other infective diseases

Small-pox has its maximum prevalence in May, but is excessive Jan. to June. Whooping Cough „ „ „ Dec. to May.

Measles commonly has two seasonal maxima, in June and December with intervening minima.

=Causes of Epidemics.=—Measles recurs in the large towns of England every alternate year. Other infective diseases occur at less regular intervals. The recurrence of epidemics is not solely due to personal infection and the accumulation of a population at susceptible ages. There are longer =cycles= of the causes of which but little is known. Thus scarlet fever has been shown by Longstaff and Gresswell to become epidemic chiefly in dry years; and I have shown that diphtheria and rheumatic fever become widely epidemic under the same conditions, diphtheria becoming so only when a series of dry years occur in immediate succession.