European population grew only slowly during the period 1200-1700; factors include disease and wars. Human feces and rotting animal remains were not sequestered and often contaminated drinking water. Cities were so filthy that more people died in them than were born. About a third of children died in infancy, many from abandonment and lack of care during wet-nursing. Children that survived were subjected to harsh discipline to control their tendency to sin. Ineffective and even harmful treatments, like blood-letting, were all that medicine could offer. Starting with Newton's Principia (1687) and the Enlightenment (eighteenth century), scientific attitudes began replacing religious ones: the biological and physical world became objects of study. Sanitation, hygiene and public health improved. Inoculation and vaccination were developed. The Industrial Revolution began. As death rates fell, population rose. While most believe that an increasing population is good, Malthus worries that population can grow faster than the food supply, trapping people in subsistence misery.