Measles

Like smallpox, measles in the New World was linked to the domesticated mammals of the Old World. As the Aztecs and other indigenous people became increasingly exposed to foreign animals, they quickly contracted the highly contagious disease. While the virus, which devastated the Valley of Mexico in 1531-1532, killed many, it covered only a small area of land; this disease was so contained because it could only be spread through contaminated air, exhaled by a sick one.