The colonies joined together for the first Continental Congress, they needed to be one "country" who went to Britain, not 13 unique ones. The British considered America to be an approach to pay off their war obligations, they considered them to be cash and a place to push around. They began giving them assesses yet did not give them portrayal in Parliament.
The reason Dysentery and so many other diseases were able to spread so rapidly through both armies was primarily because of a lack of sanitation practices and contaminated water.