Fever can be produced with similar symptoms by toxic products of different origin, as from small elements of the vegetable kingdom like bacteria, certain plants, and even fruits, as is shown by the urticaria which follows in some persons after eating strawberries. Certain minute elements of the animal kingdom have a similar power, such as protozoa like trypanosomes, and we may also instance certain kinds of animal food like oysters in certain persons, the poison of snakes, and certain insects like tarantulas and scorpions; also certain minerals like arsenic and phosphorus can produce fever. Besides these poisons coming from without, fever with similar symptoms can also be produced by poisons formed within our body by the hyperactivity of a gland—the thyroid. When so many different poisons produce the same result it lies near to suppose that they do this by means of the same agency, which, according to the aforesaid observations, is very probably a thyroid hyperactivity. The _modus operandi_ of all these agents is well studied in Sajous’s work, to which the reader is referred. As is well known, a condition of hyperactivity of the thyroid may be followed by its exhaustion, and thus Graves’s disease may often be followed by myxœdema, i.e., athyroidia. In the same way the hyperactivity of the thyroid gland in infectious diseases may also be followed by its exhaustion and a myxœdematous condition. Even complete myxœdema most frequently appears after a previous infectious disease—a fact recognized by the earliest English authors on this disease. Accordingly, it is not surprising if an infectious disease like trypanosomiasis is followed by a condition like sleeping sickness, which, as we have shown at the German Congress for Internal Medicine in 1905, presents all the clinical symptoms of, and identical anatomico-pathological alterations of the central nervous system noted in, myxœdema. On the other hand, trypanosomiasis presents all the typical symptoms of Graves’s disease. In syphilis also, after the fever with eruptions in the secondary stage, in which we not infrequently see, especially in women, a swelling of the thyroid, we find in the tertiary stage many symptoms of a condition of myxœdema or hypothyroidia, and with the iodine treatment we add to the blood the main element of the thyroid gland. Iodine is also especially active, if not given in too large doses, in exciting thyroid activity, and sometimes it even provokes Graves’s disease.