Prions, which cause diseases such as Creutzfeldt-Jakob and BSE (commonly called mad cow), are pretty scary stuff. They don't have any genetic material; instead, they're made from a protein that's normally produced by the brain. Like something out of science fiction, they adopt a distinct conformation, and then induce all the other proteins around them to adopt the same structure, gradually creating a tangle that's fatal to brain cells. Expose a healthy animal to the prions of a sick one, and the diseased form will gradually take over.

As if that weren't scary enough, evidence has been building that there are different strains of prions, presumably caused by slightly different protein structures, that vary in their ability to attack different hosts, the speed at which they progress, and their sensitivity to drugs that alter protein processing. New research published in Science shows that it's possible to convert one strain to another.

So, for example, the researchers were able to take a strain that was sensitive to a drug and grow it for a number of generations in the presence of the drug. A small fraction of the prions appear to be drug resistance at the start, and these become the majority after a number of generations. In the same way, it's possible to shift a population so it grows in better on different cells, or even better in cultured cells instead of the brain.

If all of that sounds familiar, it should—this is precisely the behavior we call evolution when it happens to a living organism or virus.

The researchers propose that there are a number (possibly more than a dozen) of low-energy potential prion structures that are separated by higher energy barriers. When a prion converts a normal protein, it typically forces it into the same structure as itself, but at a low probability, other variant structures result. The population of these variants can then expand or contract based on selective pressures.

Science, 2009. DOI: 10.1126/science.1183218