As far as terrifying medical issues go, prion diseases have got to be way up there on the list. Prions are far too tiny to see, they absolutely destroy brains and, so far, there is nothing doctors can do to stop them.

In an effort to change that, scientists are working to better understand how these mysterious particles cause such extreme devastation. Now, a group of researchers at the University of Edinburgh has revealed one of the prion's big secrets: how they get into the brain. It turns out that one of the immune system's defense mechanisms actually acts as a gateway to the brain for prions, according to the new paper in the Journal of Virology.

Prions are tiny disease-causing particles that are responsible for fatal diseases such as kuru and variant Creutzfeld-Jakob disease – the human version of mad cow disease. They are neither bacterial nor viral, nor are they living organisms at all. They are proteins gone awry.

Proteins are elemental components of life. The human body alone contains roughly 2 million different proteins.

What's so dangerous about the malformed proteins known as prions is that they are the protein equivalent of zombies. They can cause other proteins – particularly those in the brain – to become toxic prions, too. Once prions start this cascade of protein deformation, the brain quickly becomes filled with holes until there is not enough intact brain tissue left for the human, cow or other creature to survive. 

Prions do damage in the brain, yet they often come from food. Until this new study, no one was sure how prions got from the gut to the brain.

By examining prion-infected mice, the researchers determined that prions first build up in the small intestine. Specifically, they exploit structures called Peyer's patches that the immune system uses to fend off foodborne bugs.

Since this seems to be a critical early step of prion infection, the researchers hope that their work will help make it possible to detect prion diseases early enough to stop them. It is estimated that as many as one out of every 2,000 people in the U.K. carries infectious prions in their bodies but show no signs of disease.