_Streptococcus pyogenes_ is found in spreading suppurations and in very acute abscesses. About 16% of acute abscesses contain streptococci. It is easily killed by boiling, and can be destroyed by carbolic acid and by corrosive sublimate. The streptococcus of erysipelas is thought to be identical with the streptococcus pyogenes, but their difference in action is believed to be due to difference in virulence induced by external conditions and by the state of the tissues of the host. The coccus of erysipelas is larger than the ordinary form of streptococcus pyogenes, and infection takes place through a wound, often a very trivial one, or through a mucous membrane. The organism multiplies in the small lymph channels. The streptococcus may cause suppuration in erysipelas, mixed infection not being necessary to cause pus to form. The _gonococcus_ of Neisser is found both inside and outside of pus cells and mucous cells. The gonococci cannot be cultivated upon ordinary media, but grow best upon human-blood serum. Gonococci stain easily and are readily decolorized by Gram’s method. The _bacillus coli communis_, or the bacillus of Escherich, is invariably found in the fæces. It is believed by many observers to be the cause of appendicitis, peritonitis, and abscesses about the intestine. In cases of appendicitis we can rarely get a pure culture of Escherich’s bacillus, but usually find also streptococci and staphylococci. The _bacillus of typhoid fever_ (Eberth’s bacillus) is responsible for some cases of gangrene, for some of embolism and for not a few bone and joint diseases. The _bacillus tuberculosis_ (Koch’s bacillus), the cause of all tubercular processes, is met with especially in dusty air which contains the dried sputum of victims of tuberculosis. This infected air is the chief means of its transmission, though it may be conveyed by the milk of tubercular cows and by the meat of tubercular animals. Wounds may open a gateway for infection. The _bacillus tetani_ (Nicolaier’s bacillus), an aerobic organism, is found especially in the soil of gardens, in the dust of old buildings, in street dirt, and in the sweepings of stables. Spores develop at the ends of these bacilli. This organism is capable of producing toxins of deadly power. Its spores are hard to kill. CHAPTER III +ASEPSIS AND ANTISEPSIS+