The idea that life can arise from spontaneous generation has been widely disproven ever since Louis Pasteur famously showed how microorganisms in broth arose only from other living things. That's one of the reasons scientists were so surprised to witness prions — infectious proteins that cause mad cow disease, scrapie and Creutzfeldt-Jakob disease — that appear to spontaneously generate out of otherwise uninfected, healthy brain tissue, according to Nature.

Prions can propagate themselves but are not considered living organisms because they do not self-replicate. Even so, their spontaneous generation is a curious phenomenon, especially since they are the root cause of such diseases.

"It took a while before we could convince ourselves this was a real phenomenon," said one of the study's authors, John Collinge, who heads up the Department of Neurodegenerative Disease at University College London.

Researchers were working on a mouse version of mad cow disease in Collinge's lab when the phenomenon occurred. Because prions readily bind to steel wires, metal surfaces can be used to detect the presence of prions, infect brains in the lab for study and, ideally, be used for prion decontamination.

"What we were doing was trying to develop a very sensitive assay for prion detection on a metal surface, so we could use that in prion decontamination," said Collinge.

The surprise came when some of the wires coated with uninfected mouse brain, the study's controls, ended up testing positive for the deadly prions.

To ensure that the control samples were not accidentally contaminated, researchers performed the test 16 times. Nine of those experiments had controls which tested positive. They even tried the experiment in another laboratory which had never been used for prion work before, and used new equipment. Still, positive results appeared.

The mysteriously appearing prions from the positive control samples were also unique in that they caused slightly different disease symptoms, when placed in the brains of mice, from those produced by the prions normally used in the laboratory.

"In the beginning it was pretty hard to believe. We spent years repeating the experiment under more and more strenuous circumstances," said co-author Charles Weissmann, who studies prion biology at Scripps Florida in Jupiter.

Mad cow disease is a transmissible spongiform encephalopathy (TSE), or prion disease, which infects cattle and causes spongy holes to develop in the brain. The disease is called scrapie in goats and sheep, and when they infect humans, TSE's take the form of Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, kuru, and some other nervous system disorders. The diseases are highly infectious and can be transmitted from eating tainted meat.

Researchers have also suggested the possibility that the metal surface used in the experiments somehow catalysed the formation of the prions. To determine whether the metal wire could have such an effect is the next step in deciding whether genuine spontaneous generation is happening.