Soon after Europeans and enslaved Africans arrived in the New World, bringing with them the infectious diseases of Europe and Africa, observers noted immense numbers of indigenous Americans began to die from these diseases. One reason this death toll was overlooked is that once introduced, the diseases raced ahead of European immigration in many areas. The disease killed a sizable portion of the populations before European written records were made. After the epidemics had already killed massive numbers of natives, many newer European immigrants assumed that there had always been relatively few indigenous peoples. The scope of the epidemics over the years was tremendous, killing millions of people—possibly in excess of 90% of the population in the hardest-hit areas—and creating one of "the greatest human catastrophe in history, far exceeding even the disaster of the Black Death of medieval Europe", which had killed up to one-third of the people in Europe and Asia between 1347 and 1351.