Rabies

Immune System 

 

The rabies virus has an almost 100% fatality rate, this is somewhat due to the fact that it is undetectable for such a long time. The virus attacks the nervous system through the nerves, and essentially hides from the immune system in the nerves. When the immune system finally realizes the virus is there, it is too late to stop it. The macrophages don't usually check neurons for viruses, and that's why the rabies virus gets through undetected.