Contact with European diseases such as smallpox and measles killed between 50 and 67 per cent of the Aboriginal population of North America in the first hundred years after the arrival of Europeans. Some 90 per cent of the native population near Massachusetts Bay Colony died of smallpox in an epidemic in 1617–1619. In 1633, in Plymouth, the Native Americans there were exposed to smallpox because of contact with Europeans. As it had done elsewhere, the virus wiped out entire population groups of Native Americans. It reached Lake Ontario in 1636, and the lands of the Iroquois by 1679. During the 1770s, smallpox killed at least 30% of the West Coast Native Americans. The 1775–82 North American smallpox epidemic and 1837 Great Plains smallpox epidemic brought devastation and drastic population depletion among the Plains Indians. In 1832, the federal government of the United States established a smallpox vaccination program for Native Americans (The Indian Vaccination Act of 1832).

Answer this question, if possible (if impossible, reply "unanswerable"): Who were the Native Americans exposed to smallpox because of?
Europeans