The areola, which should be bright red, may be purple, and may extend a long distance from the vesicle. The pustule may be hemorrhagic or may be filled with greenish pus; in this case there is probably a mixed infection. Sometimes instead of a vesicle there appears a hard elevated nodule, in color like a red raspberry. With this there is usually no areola, and no constitutional symptoms develop. The growth is usually an evidence of poor virus. It may persist for some time before absorption. The course may be abortive,--_i.e._, the vesicle does not develop completely; pustulation comes early and the crust is shed and the scar formed before the end of the second week. This course is normal though not invariable in revaccinations. The scar may be poorly marked, even when the vaccination has run a typical course. *Complications.*--The most frequent complications are infections and eruptions. An infection may be, of course, of many sorts. It may be, for example, the streptococcus of erysipelas, or the bacillus of tetanus, but it is oftenest a skin coccus. These infections may be introduced with the virus, with the instrument, or later through wounds in the vesicle or pustule. Erysipelas and tetanus following vaccination are exceedingly rare, and it has never been shown that in a case of tetanus the germ was inoculated at the time of vaccination. Eruptions are probably usually due to a chemical irritation produced by the development of the vaccinia; they are analogous to the eruptions following the injection of antitoxine and the ingestion of various drugs. They vary in appearance, sometimes resemble the eruption of measles or of scarlet fever, and again are urticarial; they are macular, papular, and vesicular. When a moist eczema is present there may be auto-inoculation of the pock on the affected area and a general confluent vaccine eruption appear. *Immunity.*--The immunity against smallpox, or vaccinia, produced by vaccination is of gradual growth, and is not complete until the period of suppuration, about the beginning of the second week. Natural immunity is said to exist, and is probable, but it is exceedingly rare. Vaccination of a pregnant woman rarely, if ever, confers immunity on the fœtus.