After Montgomery’s combined force moved downriver on 3-4 December 1775 and took up positions on the Plains of Abraham, just west of Quebec City, the Patriot troops established artillery positions to bombard the walled city. Lacking a sufficient number of heavy guns to make much headway against the stone ramparts and facing an outbreak of smallpox, Montgomery decided to launch a bold but risky two-pronged ground attack on Quebec during a snowstorm on New Year’s Eve. The assault failed and a British cannonball took Montgomery’s life. General Wooster summarized events in a report to Congress.
"The small pox broke out in Montgomery’s army a few days before the assault. The unfortunate determination to attack the lower instead of the upper town, was a main cause of the failure. He doubts whether, if Campbell had not retreated, he could have carried the lower town, and if he had carried it, they must notwithstanding have been made prisoners by the upper town. After they had been out a while it is said their guns were so wet, not one in 10. would go off. These men had been obliged to make a circuit of miles before they could get at the place of attack. After this about 3 weeks the small pox became pretty general in the army and with their want of ammunition prevented any subsequent attack. Some whole regiments down. Were between 1900 men and 2000. Of these about 900 fit for duty. "
General Wooster’s Testimony Concerning the Canadian Campaign, 4 July 1776www.founders.archives.gov