People in the Old World, because they had lived in settled societies, in close proximity to animals, and with dense populations, developed many highly contagious diseases. These included smallpox, measles, typhus, and plague, among many others. These diseases tended to hit and spread quickly, killiing large amounts of people. But they also left people that contracted them, but didn't die with antibodies that prevented them from contracting the disease again. Indians, having never been exposed to them, had no immunity to them. This was the result of a number of factors, including the fact that they did not have many domesticated animals and that they had not lived in dense populations long enough to develop deadly epidemic diseases. While Indians probably gave conquerors syphillis, the exchange of pathogens that accompanied contact was otherwise far worse for natives, who died in unfathomable numbers, than for Europeans.