Certain bacteria are disease-producing or =pathogenic=. The largest of these is the _Bacillus of Anthrax_, a disease common in sheep and oxen, and sometimes communicated to man. This bacillus is 1·2 µ thick and 6 to 8 µ long. When an animal dies of this disease it should be buried without cutting the skin. When exposed to the air this bacillus forms minute spores, very difficult to destroy. They may live for several years in pits in which animals dying from anthrax have been buried. Butchers have died when inoculated through cracked fingers when dressing the carcase of a cow which has had anthrax. Similarly men handling the hides of such animals may be inoculated, either with a form of disease in which rapid blood-poisoning is produced, or with a malignant carbuncle, from which recovery is possible if it be treated promptly. Wool sorters of mohair wool are very liable to suffer from a fatal form of pneumonia due to the dust from wool derived from animals which have died from anthrax (page 107). This disease gives a good instance of possible _attenuation of virus_, of which another example is seen in small pox (page 293). Pasteur grew anthrax bacilli in broth at a temperature of 110° Fahr. At this temperature the bacilli multiplied by division, and no spores were formed. By repeatedly sub-culturing after the bacilli had become old (_i.e._ by putting minute quantities of the growth into fresh broth) and exposing to air, he obtained anthrax bacilli which were only slightly virulent, only producing slight constitutional disturbance when inoculated, _i.e._ injected under the skin of sheep, and yet protected them against ordinary infection by anthrax. Other methods of attenuation of virus have been discovered. For instance the growth of the bacillus in the presence of a feeble antiseptic, or passing it through the circulation of an animal which is relatively insusceptible to the particular bacillus has this effect. Other important pathogenic bacteria will be considered later (pages 298 to 398). It is only necessary here to mention that suppuration, erysipelas, puerperal fever, and a number of forms of blood-poisoning are due to the invasion of the system by =cocci=. A single round cell (commonly not more than 1∕25000 inch in diameter) is called a _micrococcus_. When in pairs as in the micro-organism causing pneumonia they are called _diplococci_; when in chains, _streptococci_ (_i.e._ twisted); when in masses, _staphylococci_. When cocci and other micro-organisms are kept out of wounds, healing occurs without suppuration; this is the principle of the antiseptic and aseptic methods of treating wounds (pages 106 and 110). The question of immunity is discussed on page 288.