Some of these imply direct contact--such, for example, as kissing, the digital examination of syphilitic patients by doctors or nurses, or infection of the surgeon's fingers while operating upon a syphilitic patient. In suckling, a syphilitic wet nurse may infect a healthy infant, or a syphilitic infant may infect a healthy wet nurse. In other cases the infection is by indirect contact, the virus being conveyed through the medium of articles contaminated by a syphilitic patient--such, for example, as surgical instruments, tobacco pipes, wind instruments, table utensils, towels, or underclothing. Physiological secretions, such as saliva, milk, or tears, are not capable of communicating the disease unless contaminated by discharge from a syphilitic sore. While the saliva itself is innocuous, it can be, and often is, contaminated by the discharge from mucous patches or other syphilitic lesions in the mouth and throat, and is then a dangerous medium of infection. Unless these extra-genital sources of infection are borne in mind, there is a danger of failing to recognise the primary lesion of syphilis in unusual positions, such as the lip, finger, or nipple. When the disease is thus acquired by innocent transfer, it is known as _syphilis insontium_. #Stages or Periods of Syphilis.#--Following the teaching of Ricord, it is customary to divide the life-history of syphilis into three periods or stages, referred to, for convenience, as primary, secondary, and tertiary. This division is to some extent arbitrary and artificial, as the different stages overlap one another, and the lesions of one stage merge insensibly into those of another. Wide variations are met with in the manifestations of the secondary stage, and histologically there is no valid distinction to be drawn between secondary and tertiary lesions. _The primary period_ embraces the interval that elapses between the initial infection and the first constitutional manifestations,--roughly, from four to eight weeks,--and includes the period of incubation, the development of the primary sore, and the enlargement of the nearest lymph glands. _The secondary period_ varies in duration from one to two years, during which time the patient is liable to suffer from manifestations which are for the most part superficial in character, affecting the skin and its appendages, the mucous membranes, and the lymph glands. _The tertiary period_ has no time-limit except that it follows upon the secondary, so that during the remainder of his life the patient is liable to suffer from manifestations which may affect the deeper tissues and internal organs as well as the skin and mucous membranes.