Just a minor qualifier on many of the examples here: humans (and all multicellular organisms, but we're particularly bad) evolve orders of magnitude more slowly than the microorganisms that cause disease (since we breed slower). So when it happens that a segment of the population survives a disease while another dies, it's not necessarily because some have "evolved resistance". The same tends to happen with even new diseases. We do have evolved immunity but it is by necessity broad, so a segment of the population might be immune because they're descendents of survivors of a similar disease, or it may be entirely coincidental - and that coincidence can go both ways (such as survivors of Plague now being more susceptible to auto immune diseases).