I think you have to be careful in distinguishing between what some of the colonists and British officers would have like to have done, and what they actually could do. While there is evidence that they thought of the idea of intentionally spreading smallpox, there is no direct evidence that they actually carried out their idea.

Smallpox is generally spread from a person who is actually breaking out with the smallpox, and not from a piece of clothing or blanket. Is it possible to carry the infection on a blanket or an article of clothing, probably...Would it be an effective delivery system, probably not. Considering that the Europeans did not understand disease the way we do today, much less delivery systems, it would have been an adventure into the unknown to wage such warfare. After watching American Indians die in large numbers from disease during the 17th and 18th centuries, however, the idea had to be tempting. The fact that the British had a smallpox ward to supply the blankets meant they had active cases of smallpox in the ward, and presumably, in the area. They did understand the need to isolate and quarantine- it would have been a further risk to their own people to have spread smallpox further.

Alfred W.Crosby has written several books on the biological and ecological effects of colonization by the Europeans on the Americas. In "Ecological Imperialism- The Biological Expansion of Europe, 900-1900" he addresses these legends in a footnote (n38, p.345): "The colonists certainly would have liked to wage such a war [bacteriological warfare] and did talk about giving infected blankets and such to the indigenes, and they may have even done so a few times, but by and large the legend is just that, a legend...disease was, in practical terms, people who were sick- an awkward weapon to aim at anyone. As for infected blankets, they might or might not work. Furthermore, and most important, the intentionally transmitted disease might swing back on the white population."

Crosby's book deals with other major issues besides disease and illness, which is why he mentions the legend in a footnote and not in a separate chapter. Along with disease, the importation (intentional and unintentional) of animals, plants, weeds, and fruits did a lot to change the landscape of America.

John