The period of protection afforded by a successful vaccination is uncertain, because it varies with different individuals. In a general way immunity for about four or five years is thus secured; ten or twelve years after vaccination the protection is certainly lost and smallpox may be then acquired. Every individual should be vaccinated between the second and third month after birth, and between the ages of ten and twelve, and at other times whenever an epidemic threatens. An unvaccinated person should be vaccinated and revaccinated, until the result is successful, as immunity to vaccination in an unvaccinated person is practically unknown. When unsuccessful, the vaccine matter or the technique is faulty. A person continuously exposed to smallpox should be vaccinated every few weeks--if unsuccessful, no harm or suffering follow; if successful, it proves liability to smallpox. A person previously vaccinated successfully may "take" again at any time after four or five years, and, in event of possible exposure to smallpox, should be revaccinated several times within a few weeks--if the vaccination does not "take"--before the attempt is given up. An unvaccinated person, who has been exposed to smallpox, can often escape the disease if successfully vaccinated within three days from the date of the exposure, but is not sure to do so. Diseases are not introduced with vaccination now that the vaccine matter is taken from calves and not from the human being, as formerly. Most of the trouble and inflammation of the vaccinated part following vaccination may be avoided by cleanliness and proper care in vaccinating.