On 8 May 1776, General Thomas notified Washington that his army in Canada was simply too small and too sick to continue the siege of Quebec City. The withdraw from Quebec, he wrote, must begin at once.
"Immediately on my arrival at the camp before Quebec, which was on the first instant, I examined into the state of the army, and found by the returns there were 1900 men. Of this number only a 1000 were fit for duty, Officers included; the remainder were Invalids, chiefly confined with the small pox. Three hundred of the effective were Soldiers whose inlistments expired the 15th Ultimo, many of whom peremptorily refused duty, and all were very importunate to return home; and two hundred others engaged for the year, had received the infection of the small pox by inoculation, & wou’d in a short time be in the Hospitals. Several posts were necessary to be supported by the small number able to do duty, at such distances from each other, that by means of Rivers and other obstructions, not more than 300 men could be rallied to the relief of any one post"
General Thomas to General Washington, 8 May 1776www.founders.gov