+Contagious Recurrences or Relapses.+—­The duration of the infectious period in untreated cases and the proportion of infectious lesions in a given case vary a good deal and both may be matters of the utmost importance.  Some persons with syphilis may have almost no recognizable lesions after the chancre has disappeared.  Others under the same conditions may have crop after crop of them.  There is a kind of case in which recurrences are especially common on the mucous or moist surfaces of the mouth and throat, and such patients may hardly be free from them or from warty and moist growths about the genitals during the first five years of the disease, unless they are continuously and thoroughly treated.  Irritation about the genitals and the use of tobacco in the mouth encourage the appearance of contagious patches.  Smokers, chewers, persons with foul mouths and bad teeth, and prostitutes are especially dangerous for these reasons.

+Average Contagious Period.+—­It is a safe general rule, the product of long experience, to consider a person with an untreated[10] syphilis as decidedly infectious for the first three years of his disease, and somewhat so the next two years.  The duration of infectiousness may be longer, although it is not the rule.  It must be said, however, that more exact study of this matter since the germ of syphilis was discovered has tended to show that the contagious period is apt to be longer than was at first supposed, and has taught us the importance of hidden sores in such places as the throat and vagina.

    [10] The control of infectiousness in syphilis through treatment is
    considered in the next chapter.

    [Illustration:  FRITZ SCHAUDINN [1871-1906]

    (From the “Galerie hervorragender Aerzte und Naturforscher.” 
    Supplement to the Muenchener med.  Wochenschrift, 1906.  J. F. Lehmann,
    Munich.)]

+Individual Resistance to Infection.+—­The contagiousness of untreated syphilis is influenced by two other factors besides the mere lapse of time.  The first of these is the resistance or opposition offered to the germ by the person to whom the infection is carried.  The second is the feebleness of the germ itself, and the ease with which it dies when removed from the body.  In regard to the first of these factors, while natural resistance to the disease in uninfected persons is an uncertain quantity, it is very probable that it exists.  It is certain that the absence of any break in the skin on which the germs are deposited makes a decided difference if it does not entirely remove the risk of infection.  A favorable place for the germ to get a foothold is a matter of the greatest importance.  When, however, it is remembered that such a break may exist and not be visible, it is evident that little reliance should be placed on this factor in estimating the risk or possibility of infection.