The toxin theory does not fully account for those cases that occur suddenly, without any manifest pre-existing disease, and as the result apparently of cold and chill. On the other hand, it is only a very small proportion of horses exposed to the same degree of cold and chill that contract petechial fever, and it might well be surmised that in these few an unknown focus of suppuration or other lesion existed prior to the chill or that toxins having the requisite devitalizing and vaso-dilating properties had been absorbed from fermentations in the bowels or elsewhere. The mere exposure is harmless to the very great majority of subjects. In any case it must be accepted that the debility and impaired local innervation, nutrition and function, that attend on the exposure to cold and toxins must be looked on as potent contributing causes. The predilection of the swellings for dependent parts (limbs, venter, face) shows the influence of gravitation and congestion. Whether there is present any special microbe which has yet eluded discovery, but which is the main pathogenic factor, must be left to the future to decide. In cases that appear to be due to cold or chill alone, the disease is held to be _primary_; in those following on another affection, _secondary_. Among the diseases on which petechial fever supervenes as a secondary affection contagious inflammatory affections of the lungs and air passages hold a bad preëminence. Strangles, influenza and contagious pneumonia, about in the order named, are especially causative factors or occasions of petechial fever. Among the other affections on which it supervenes may be named pharyngitis, abscess of the nasal sinuses, hepatic, renal and other internal abscesses, acute coryza, laryngitis, or bronchitis, enteritis, abortion, aggravated grease, suppurating wounds of the skin, infective abrasions by harness, suppurating sores after firing, infective arthritis with open joint, amputation of the tail, and castration. _Lesions._ In certain cases these may be largely confined to petechiæ and slight blood extravasations, which are distributed very generally throughout the tissues, but show especially in the skin, subcutis, mucosa and submucosa of the nose, eyes, pharynx, guttural pouches, larynx, trachea, bronchia, mouth, stomach, intestine, bladder, vagina and womb; also in and on the lungs, pleura, pericardium, heart, liver, spleen, kidneys, peritoneum, pancreas, ovaries, bones, lymph glands, brain and nerves. The largest extravasations are liable to be in the softest tissues, and in the lungs they may reach the size of the closed fist, though usually they vary from a mere spot up to this. The spleen is sometimes engorged even to rupture. Beside the extravasations, and associated with them in position, and probably largely as an effect of them, there is more or less serous effusion infiltrating the tissues, congestions, suppurations, degenerations, and necrotic changes.