Returning to England after further travels in France and Germany, he was incorporated M.D. at Cambridge, and practised apparently at Cambridge, Norwich, and Shrewsbury, with such success that he was appointed physician to Edward VI., an appointment he continued to hold under Queens Mary and Elizabeth. On the 22d December, 1547, he was admitted a Fellow of the College of Physicians, and in 1550 became an Elect, in 1552 Censor. In the latter year appeared his English treatise on the Sweating Sickness, which had broken out at Shrewsbury in 1551. This was afterwards enlarged and published in Latin. “The Boke or Counseill against the Sweatyng Sicknesse,” was dedicated by Dr. Caius to William, Earl of Pembroke. The dedication begins thus: “In the fearful time of the sweat, many resorted unto me for counsel, among whom some being my friends and acquaintance, desired me to write unto them some little counsel how to govern themselves therein.... At whose request at that time, I wrote divers counsels so shortly as I could for the present necessity, which they both used and did give abroad to many others, and further appointed in myself to fulfil the other part of their honest request for the time to come. The which the better to execute and bring to pass, I spared not to go to all those that sent for me, both poor and rich, day and night. And that not only to do them that ease that I could, and to instruct them for their recovery; but to note also thoroughly the cases and circumstances of the disease in divers persons, and to understand the nature and causes of the same fully, for so much as might be.” A certain conceit is evident throughout the brief treatise, as when he describes his early translations from Latin into English, and partially apologises for writing in English, then gives an account of the life and writings of his friend, William Framingham, a fellow-townsman of his who died young. The description of the disease which he gives indicates a very acute rheumatic affection, inasmuch as perspirations of disagreeable odour, acute pains in the limbs, delirium, quick and irritable pulse, &c., were prominent among them. It is notable how little medical science was progressing beyond Galenic principles. Dr. Caius says, “This disease is not a sweat only, but a fever in the spirits by putrefaction venomous, with a fight, travail, and labour of nature against the infection received in the spirits, whereupon by chance followeth a sweat, or issueth an humour, compelled by nature, as also chanceth in other sicknesses which consist in humours.” Still, a glimpse of truth is shown in the view expressed that “our bodies can not suffer anything or hurt by corrupt and infective causes, except there be in them a certain matter prepared, apt and like to receive it, else if one were sick, all should be sick.”