In the human subject, multiplication in the blood-stream does not occur to any great extent. In some general acute pyogenic infections, such as osteomyelitis, cellulitis, etc., pure cultures of staphylococci or of streptococci may be obtained from the blood. In pneumococcal and typhoid infections, also, the organisms may be found in the blood. It is by the vital changes they bring about in the parts where they settle that micro-organisms disturb the health of the patient. In deriving nourishment from the complex organic compounds in which they nourish, the organisms evolve, probably by means of a ferment, certain chemical products of unknown composition, but probably colloidal in nature, and known as _toxins_. When these poisons are absorbed into the general circulation they give rise to certain groups of symptoms--such as rise of temperature, associated circulatory and respiratory derangements, interference with the gastro-intestinal functions and also with those of the nervous system--which go to make up the condition known as blood-poisoning, toxæmia, or _bacterial intoxication_. In addition to this, certain bacteria produce toxins that give rise to definite and distinct groups of symptoms--such as the convulsions of tetanus, or the paralyses that follow diphtheria. _Death of Bacteria._--Under certain circumstances, it would appear that the accumulation of the toxic products of bacterial action tends to interfere with the continued life and growth of the organisms themselves, and in this way the natural cure of certain diseases is brought about. Outside the body, bacteria may be killed by starvation, by want of moisture, by being subjected to high temperature, or by the action of certain chemical agents of which carbolic acid, the perchloride and biniodide of mercury, and various chlorine preparations are the most powerful. #Immunity.#--Some persons are insusceptible to infection by certain diseases, from which they are said to enjoy a _natural immunity_. In many acute diseases one attack protects the patient, for a time at least, from a second attack--_acquired immunity_. _Phagocytosis._--In the production of immunity the leucocytes and certain other cells play an important part in virtue of the power they possess of ingesting bacteria and of destroying them by a process of intra-cellular digestion. To this process Metchnikoff gave the name of _phagocytosis_, and he recognised two forms of _phagocytes_: (1) the _microphages_, which are the polymorpho-nuclear leucocytes of the blood; and (2) the _macrophages_, which include the larger hyaline leucocytes, endothelial cells, and connective-tissue corpuscles.