http://www.pbs.org/wnet/secrets/previous_seasons/case_plague/index.html
I highly recommend checking out the excellent documentaries that have been done about this. It started when a gay man in the US, who engaged in a lot of high risk sex with HIV infected men, showed no sign of HIV after years of exposure.
Eventually it led researchers back to the small, central England town of Eyam, and a genetic mutation some of the people had during the black death. If one of their parents had the mutation, they would get sick, but recover. But if both their parents had the mutation, they would not get sick at all.
It turned out that the mutation made their cells “slick”, so that viruses could not latch on to them. If they couldn’t establish a connection, they could not enter the cell. And it applied equally well to HIV.
In describing influenza, for example, H1N1, the “H” factor, which stands for Hemagglutinin, is simply how the virus attaches to and enters the cell. The “N” factor (Neuraminidase) is how after reproducing, the millions of new viruses punch their way out of the cell back into the bloodstream, before the dead cell is destroyed by the body.
There are about 17 known types of the “H” factor, but only three of them (H1,H2, and H3) currently attack human cells. H5 would be a new one, making H5N1 extra deadly. Only two kinds of “N” factor are specific to human disease, N1 and N2.
In addition to genetic mutation, there are some chemicals that can also make cells “slicker”, but it is not known if these are universal to viruses. For example, a lot of cranberry juice has long been consumed against bladder infections. Apparently it can make cells slick against some viruses and bacteria that cause both bladder infections and diarrhea.
Some other fruit juices may have a similar effect. However, this is still relatively new information, so it is not yet known whether it would affect other viruses and bacteria.
I have a natural immunity to killer robots with nukes.