Sometimes a scientific discovery shakes scientists' confidence, leading them to question whether they truly understand the fundamental rules of nature. This is exactly what prions have done to scientists' understanding of the basic rules of infectious diseases. Prions cause disease, but they are not viruses, bacteria, fungi, or parasites. They are simply proteins, and proteins were never thought to be contagious on their own. Organisms are contagious, proteins are not. Or, at least, they never were. Prions entered public consciousness during the mad cow epidemic that hit England in 1986. For decades, however, scientists had searched for unusual and atypical infectious agents that they suspected caused some puzzling diseases that could not be linked to any of the organisms "normal" infectious diseases. . One possibility was that slow viruses – viruses that have spent decades wreaking havoc on their hosts – could be the culprit, and these supposed viruses were not only slow to multiply, but were also difficult to isolate. Now researchers are coming, albeit reluctantly, to accept the shocking fact that naked proteins can be infectious. Prions enter cells and apparently convert normal proteins found inside cells into prions just like them. Proteins in normal cells have all the same "parts" as prions - notably the same amino acid building blocks - but they fold differently. They are very similar to the toy "Transformers" that were around in the 80s. They could transform into different shapes without adding or subtracting anything. Prions enter brain cells and there convert the normal cellular protein PrPC into the prion form of the protein, called PrPSC. When normal cellular proteins turn into prions, amino acids that are tightly folded into alpha-helical structures relax into looser beta sheets. More and more PrPC molecules transform into PrPSC molecules, until eventually the prions completely clog the infected brain cells. Cells misfire, malfunction, or do not function at all. In mad cow disease, for example, with brain cells firing wildly, mad cows stagger, stagger, and appear frightened—their "madness" is madness, not rage. Sheep and goats affected by scrapie disease, which is similar to mad cow disease, become so uncomfortable and itchy that they frantically rub themselves against anything they can, scraping off – hence the disease's name – much of their wool and of their fur. , infected, prion-swollen brain cells die and release prions into the tissue. These prions then enter, infect, and destroy other brain cells. And, as groups of cells die, the brain stops looking like a brain and starts looking like more