Prions absolutely terrify me. Primarily because they are so difficult to kill, er.. destroy. They are basically the zombies of infectious particles.
http://www.nature.com/news/2008/080701/full/news.2008.926.ht...
http://www.nature.com/news/2008/080701/full/news.2008.926.ht...
Open-access version: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3575082/