Why did Brian get sick from the gut cherries in the book hatchet?

In the book "Hatchet" by Gary Paulsen, Brian gets sick from the gut cherries because they are not ripe and contain toxins that his body cannot handle. He eats them in his desperation for food, unaware of their potential to make him ill. The experience serves as a harsh lesson about the dangers of foraging in the wild without proper knowledge. This incident emphasizes the importance of understanding nature and the risks associated with survival.