REPORT ON THE LAWS OF CONTAGION. S3 



US to pronounce, of any one disease, at what period it begins to 

 be infectious. Dr. Russell could not satisfy himself on this point 

 as to the plague*. The smallpox was believed by Dr. Hay- 

 garth not to be attended with contagious eflfiuvia until after the 

 appearance of the eruption, and to diffuse its poison most abun- 

 dantly when the pustules had reached the period of maturationf. 

 Scarlatina is well known to spread by infection, before the cha- 

 racteristic eruption on the skin shows itself. It is probable 

 that the infectious period is not always the same for the same 

 disease, but bears some proportion to the violence of the fever, 

 and to other circumstances. 



XXX. It has not yet been decided respecting any one disease, 

 when it ceases to be infectious. Dr. Russell coiUd not determine 

 when convalescents from the plague ceased to infect others, nor 

 when the fluid contained in the glandular abscesses was no longer 

 dangerous. Persons, recovering from smallpox, infect others so 

 long as the smallest scab is visible on the skin. Convalescents 

 from scarlatina continue to impart that disease for ten days, or 

 longer, after all the symptoms have disappeared, and even after 

 the desquamation of the cuticle I . Hence, in part, the difficulty 

 of eradicating that malady from any situation where numbers are 

 subject to it. Asiatic cholera (a disease contagious under cer- 

 tain circumstances,) emits the most active poison in its advanced 

 stage, or rather in the state of consecutive fever. The infectious 

 property of the bodies of persons who have died of that disease, 

 though testified by several writers §, requires more accurate in- 

 vestigation. If the affirmative should be established, the effect 

 may still be imputed to a poison formed during life, and only 

 exhaled after death. Infection from bodies dead of plague is 

 denied by Howard, Desgenettes, and Wittman, and the infec- 

 tious power of yellow fever is said to terminate with life. 



XXXI. It is seldom that the effects of contagious poisons, either 

 liquid or vaporous, manifest themselves immediately after being 

 received into the body. Well authenticated instances, however, 

 are not wanting of the speedy and decided operation of the effluvia 

 of plague, typhus, smallpox, &c.,when in a concentrated form. 

 But in a great majority of cases, several days or weeks (in the 

 instance of hydrophobia, even months) have elapsed, before the 

 morbid phaenomena have appeared. The period differs for dif- 

 ferent poisons, and is not always the same for the same poison. 

 It has been called the latent period of infection, the time of in ■ 

 cuhation, &c. The following intervals, though collected from 

 the best sources, are to be considered merely as approximations. 



• Russell, p. 304. t Inquiry, p. 53. 



X Blackburn, pp. 5, 14, 36. § Becker On ChoJeru 



