I don't really understand this question. Europeans were immune to smallpox because it was endemic -- they were largely infected as children, and then either died (which tons of them did), or developed lifelong immunity. There are case reports of "virgin soil" epidemics in remote villages with mortality that was similar to when it was introduced in the New World. I should mention that this principle was behind variolation, the first effective prevention of smallpox (purposefully infecting children via prick in the skin to cause a local smallpox reaction that was far less deadly). Europeans today are not immune -- as no one is -- because smallpox was eliminated in the 1970s. Well, perhaps there are still a few older adults in Bangladesh and Southern Africa who have antibodies, but their number would dwindle by the day.