Before the potato famine, also known as the Great Hunger, Ireland was the most densely populated and one of the poorest countries in Europe. Its population explosion has been attributed to the ability to feed a family almost exclusively on potatoes grown on a small parcel of land. Phytophthora infestans, a fungus causing the blight, affected only the potato. When blight appeared in Ireland, quickly destroying virtually the entire food supply of many of the poorest citizens, there was no comprehension of its cause. The potato famine led to the loss through death and emigration of one fourth of Ireland's population. A lesion to be learned is the folly of depending almost solely on one staple crop. With complementary foods available, the devastation would have been lessened.