One factor in the defeat of the American-indian civilizations was their demographic collapse. There has been an debate among researchers, that "there is no consensus as to the cause of that collapse; some give genocide as the main cause",which is very exaggerated. Some attribute it to the introduction of new diseases and a still others to a combination of both factors. Scholars now believe that, among the various contributing factors, epidemic disease was the overwhelming cause of the population decline of the Native Americans because of their lack of immunity to new diseases brought from Europe.[5] The American researcher HF Dobyns has estimated that 95% of the total population of Americas died in the first 130 years after the arrival of Columbus.[6] Cook and Borak of the University of Berkeley claim that the population in Mexico declined from 25.2 million in 1518 to 700 thousand people in 1623, less than 3% of the original population