He joined the College of Physicians in 1805, became a fellow in 1806, was appointed to give the Gulstonian lectures the same year, and was made a censor in 1808, at the comparatively early ago of thirty-six; he was a pamphleteer against the pretensions of army surgeons.
In 1811 he gave up practice in London, owing to ill-health, and resumed his full-pay rank as physician to the forces, proceeding to Jamaica.
[2] Bancroft's earliest writings were two polemical pamphlets—"A Letter to the Commissioners of Military Enquiry, containing Animadversions on the Fifth Report", London, 1808, and "Exposure of Misrepresentations by Dr. McGrigor and Dr. Jackson to the Commissioners of Military Enquiry", London, 1808—on proposed changes in the army medical department in which he contended for the then existing artificial distinctions between physician to the forces and regimental surgeon, and for the precedence of the former.
The spontaneous, autochthonous, or de novo origin of the contagia of pestilential diseases was then the generally accepted one, though the doctrine of the reproduction of a pathogen existing ab æterno had been stated, among others, by Alard Mauritius Eggerdes, a Prussian physician, for bubonic plague in 1720.
He argued, "There is no chance, nor even possibility, of thus generating anything so wonderful and so immutable as contagion, which, resembling animals and vegetables in the faculty of propagating itself, must, like them, have been the original work of our common Creator.
Bancroft explained away facts vouched for by observers such as John Pringle, Donald Monro, and Gilbert Blane.