The European colonization of the Americas forever changed the lives and cultures of the peoples of the continents. Although the exact pre-contact population of the Americas is unknown, scholars estimate that Native American populations diminished by between 80 and 90% within the first centuries of contact with Europeans. The leading cause was disease. The continent was ravaged by epidemics of diseases such as smallpox, measles, and cholera, which were brought from Europe by the early explorers and spread quickly into new areas even before later explorers and colonists reached them. Native Americans suffered high mortality rates due to their lack of prior exposure to these diseases. The loss of lives was exacerbated by conflict between colonists and indigenous people. Colonists also frequently perpetrated massacres on the indigenous groups and enslaved them. According to the U.S. Bureau of the Census (1894), the North American Indian Wars of the 19th century cost the lives of about 19,000 whites and 30,000 Native Americans.