The so-called "slow viruses" are a group of infective agents of which very little information is known. In fact, one is not sure if these agent has compositions resembling that of the viruses: all one knows is that they are communicable, with slow replication and long latent intervals in the host. Another name given to these agents is "prions" : proteinaceous and infectious. An example is scrapie, which is a disease of sheep; its incubation period is longer than 4 months, and the prion can withstand boiling, x-ray irradiation, and treatment with formalin. In 1996, bovine spongiform encephalopathy (BSE) was linked to the ingestion of beef infected with prions (the mad-cow disease). It was speculated that the prions induced changes in the conformation of certain proteins of brain cells.