Nordic Nations & HIV
I can't seem to find the original source for this theory, but it is "common" knowledge that people from Scandinavia, or the Nordic countries in general, are less susceptible to becoming infected with the HIV virus. It is thought that this is because most (if not all) of Northern Europe was never conquered by the Romans. Apparently those of us in the world (or in Europe, really) that are descendants of Romans have less protection against HIV, than those of us who 'escaped' the Roman Empire conquest. This DNA that Nordics have "lacks certain elements, which means HIV cannot bind to it as easily and is less able to infect cells. People with the mutation have some resistance to HIV infection and also take long to develop AIDS", as reported by the Telegraph.
Just like monkeys
Scientists have discovered a mechanism that may help prevent the development of AIDS in Mangabey Monkeys. Supposedly this has to do with the monkeys' ability to regenerate T-cells (type of white blood cell). Consequently, T-cells in humans, are the cells that fight of infection, and a high number of T-cells reduces one's chances of HIV. The scientist who led the study, Mirko Paiardini, believes that the new findings "have increased our understanding of the immune system and are critical to our continuing research to determine why some species are more susceptible than others to infectious diseases".
I don't know about you.. But sounds to me like Nordics are more closely related to monkey's than the rest of the world...