Like constipation, diarrhcea is a relative term. People who are regularly moved once a day would consider several movements to constitute diarrhcea. Other people are usually moved as many times a day and do not feel well when they are not. Diarrhea is a too frequent evacuation of the bowels. It may be due to many causes, which can be classified as: (a) mechanical; (b) digestive, toxic; (c) pathological - (i) bacterial, (2) nervous, lienteric, (3) catarrhal, (4) other pathological causes. Acute diarrhea is usually associated with some impurity of the food, water, or air.

Mechanical causes arise from an unusual constitution of the food. The consumption of unripe fruit, coarse vegetables, or an extra amount of indigestible cellulose, for instance in oatmeal, or irritation by mercury or some other drug, are included in this group.

Digestive causes arise from the character of the food or the digestive secretions. Indigestible foods rank amongst the commonest causes of diarrhea. Pork, veal, and similar materials set up diarrhea quite readily in some persons. The freshest of eggs will cause others to have frequent movements of the bowels, attended by the signs of an acute diarrhea; but a failure of the proper secretions of the digestive organs also causes it. This may begin in the stomach. If there is a deficiency of hydrochloric acid the food may be propelled into the bowels in a half-prepared condition. There may be a deficiency of bile, pancreatic secretion, or succus entericus, and the resulting diarrhea has been called "intestinal fermentation dyspepsia". The presence of an undue proportion of organic acids, either in the food consumed or originating from fermentation in the bowels, will give rise to diarrhea, and this condition has been called "acid jejunal diarrhea". The toxic causes of diarrhea are those of poisoning, auto-intoxication, uraemia, and septicaemia. Diarrhea due to this group of causes is sometimes salutary, and may be considered "compensatory" in character, as in the crises of Blight's disease.

Bacteria are responsible for a very large proportion of cases of diarrhea. In ptomaine poisoning, which is frequently accompanied by diarrhea, the poisons are produced by the action of bacteria. Animal foods, such as meat, game, fish, eggs, and their numerous preparations, are prone to undergo decomposition by the ordinary putrefactive organisms; but other bacteria have been found associated with animal foods in the various epidemics of food poisoning, notably B. boiulinus, B. enteritidis of Gartner, B. enteritidis sporogenes of Klein, and B. certryke of Durham. Milk, cream, ice-cream, cheese, and other substances made from milk and eggs may contain the foregoing bacteria, besides numerous streptococci and staphylococci.

It has been thought that the excessive heat of summer has much to do with the causation of diarrhea. There is no clear evidence that heat alone can cause it. When diarrhea is due to atmospheric changes, such as a chill, the attack is more often of the nature of acute intestinal catarrh, like a cold in the head. Without the action of the intestinal muscular walls diarrhea could not occur. This action may be excited by a chill. Moreover, the intestinal muscles are under the influence of nerves which are exceedingly sensitive and are excited by changes of temperature. But it is probable that the heat of summer, which is favourable to the production of bacteria, is more effective when combined with drought, dust, and other means of spreading bacteria among our food. Pathogenic organisms are distributed by air currents, dust, flies, etc. The epidemics of summer diarrhea, especially among infants, beginning in the middle of July and extending throughout August, are not essentially due to the heat. Such diarrhea is usually a bacterial disease. The faeces normally contain vast quantities of bacteria, but usually these have very little power of causing disease, or they are prevented from so doing by the healthy condition of the mucous membrane. But if the mucous membrane, irritated by improper food, becomes congested or sheds patches of epithelium, the ordinary colon bacillus is capable of causing diarrhea; it has been stated that 25 per cent of cases of diarrhea in breast-fed children are caused in that way. Uncommon micro-organisms of the group of colon bacilli have been found in many diar-rhoeal motions, whence it has been concluded that such bacteria can change their character with the circumstances and surroundings. In addition to these, various streptococci, staphylococci, the proteus bacillus, the bacillus of Shiga, and other micro-organisms gain entrance to the body by the air, or more probably by pollution of the food. One of the commonest causes of such pollution is the ordinary housefly, an insect which is most filthy in its habits, and, although a scavenger, wanders from one article to another irrespectively and pollutes whatever it touches.

Water and other fluids may be markedly contaminated by such micro-organisms. Liquids are the means of carrying bacteria of many kinds into the system. Where the gastric functions are perfect a large proportion of such bacteria are destroyed by the hydrochloric acid of the stomach, but when the organism is depressed by excessive heat and various other causes the hydrochloric acid becomes deficient or the number of bacteria is disproportionate to its disinfecting power. It was formerly thought that cholera was transmitted through the air, like infectious fevers, but the investigation by Hart showed that "we eat or we drink cholera," and the same is true of dysentery and enteric fever.

The influence of the nervous system over intestinal movement and secretion has been referred to. Many cases of diarrhea are due to this, the exciting cause being mental agitation, emotion, worry, anxiety, etc To this group probably belongs the peculiar form called "lienteric diarrhea". The common symptom of this diarrhea, to use a patient's phrase, is that "the food goes through the bowel as soon as it is eaten". This, of course, is an exaggeration, but there is some foundation for the popular description. The consumption of food always starts a peristaltic movement of the bowels which goes on at intervals until the meal is digested. But in the condition of lientery these movements are more powerful and prolonged, with the consequence that an evacuation of the contents of the colon is provoked in a short time. The exaggerated movement drives food along the bowels quickly, so that a considerable portion of it speedily arrives in the ileum in a semi-prepared form. In course of time the irritation produced by such badly prepared food may cause enteric catarrh, and the lienteric diarrhea tends to become chronic diarrhea of a catarrhal type.

The diarrhea of intestinal catarrh is the result of intestinal indigestion and fermentation in almost all chronic cases. An acute catarrhal enteritis is often due to the same causes as acute gastric catarrh and runs concurrently with it. In chronic catarrhal enteritis diarrhea frequently alternates with constipation; in other cases there may be continuous diarrhea. In addition to these causes of diarrhea there are cholera, dysentery, sprue, tuberculosis, syphilis, various ulcerations, and sometimes cancer.

The chief symptoms indicative of diarrhea are frequent stools and attacks of pain. The stools may be of an ordinary character, but loose or sloppy; they may be acid and fermented, containing some undigested food, foul smelling, green, or mixed with visible mucus. The attacks of pain vary in severity and are sometimes so intense that the skin is bathed in perspiration, while the patient rolls in agony; there is no abdominal tenderness; indeed, the pain is usually relieved by pressure or friction. If these attacks of tormina affect the colon they produce a speedy and copious evacuation; if they affect the ileum or jejunum, vomiting very commonly occurs. Diarrhea, cholera, and dysentery are most common in the summer and autumn, being promoted by heat or alternately hot and wet weather, checked by heavy rain and cold. They most frequently affect those persons whose health is broken down by disease, debility, insufficient food, or improper modes of feeding. In infantile diarrhea several important symptoms occur: collapse, loss of elasticity of the skin, and rise of temperature. The collapse is due to the pain, absence of food, depressing conditions, and, above all, to the absorption of toxins from the alimentary canal. The loss of elasticity of the skin is not fully explained, but is due to a combination of the same causes; it is a very grave sign. The stage of collapse is indicated by coldness of the surface, rapid and feeble pulse, pinched features, dark rings round the eyes, blue finger nails, sighing respiration, etc. If the patient passes through this stage it will be succeeded by a rise of temperature and a more or less febrile aspect. These cases resemble Asiatic cholera in the occurrence of a cold stage followed by a hot stage; but it is seldom that they present the rice-water stools characteristic of cholera, or the free discharge of blood so clearly indicative of dysentery.

In ptomaine poisoning there may be sickness, purging, cramp of the extremities, pains and spasms in the abdomen due to tormina, with the signs of collapse already described ; in other cases the collapse may be replaced by extreme muscular weakness, numbness of the limbs, and a feeble action of the heart, followed by muscular twitchings like those of strychnine poisoning or uraemia, and sometimes epileptiform convulsions, terminating in coma. The epidemics of food poisoning occasionally reported all have their origin in the production of ptomaines before the food was consumed. They have been connected with meat, sausages, potted meat, meat pies, ice-cream, mouldy bread, bread puddings, tinned foods, old cheese, oysters, mussels, whelks, crabs, lobsters, etc.