“ He first satisfied himself that two different forms of disease had been hitherto confounded under the term cow-pox, only one of which protected against small-pox, and that many of the cases of failure were to be thus accounted for; and his next step was to ascertain that the true cow-pox itself only protects when communicated at a particular stage of the disease. At the same time he came to the conclusion that “the grease” of horses is the same disease as cow-pox and small-pox, each being modified by the organism in which it was developed. ”

“ Towards the end of 1802 steps were taken to form a society for the proper spread of vaccination in London, and the Royal Jennerian Society was finally established, Jenner returning to town to preside at the first meeting. This institution began very prosperously, more than twelve thousand persons having been inoculated in the first eighteen months, and with such effect that the deaths from small-pox, which for the latter half of the 18th century had averaged 2018 annually, fell in 1804 to 622. ”

“ In 1818 a severe epidemic of small-pox prevailed, and fresh doubts were thrown on the efficacy of vaccination, in part apparently owing to the bad quality of the vaccine lymph employed. This caused Jenner much annoyance, which was relieved by an able defence of the practice, written by Sir Gilbert Blane. But this led him, in 1821, to send a circular letter to most of the medical men in the kingdom inquiring into the effect of other skin diseases in modifying the progress of cow-pox. ”