The correct answer is - disease, a harsh winter, and lack of food.
Colonial soldiers had it really bad during the American Revolution at Valley Forge. This was due to few things that were not taken into consideration on time and by some bad estimates, like the weather and food supplies. The weather was not by any means suitable for waging war and significantly weakened the army because the winter was very cold and with lot of snow, which led to lot of the soldiers getting sick and suffer from hypothermia. Also the estimates about the food supplies were badly made and the soldiers suffered from malnutrition which made them much weaker, and also made lots of them to get diseases.