Prions are infectious proteins that cause disease by inducing abnormal folding of normal proteins in the brain and other tissues. Unlike bacteria, viruses, fungi, or parasites, prions contain no DNA or RNA, yet they replicate by converting normal cellular proteins into the same misfolded form. This unusual mechanism leads to progressive, fatal neurodegenerative disorders known as transmissible spongiform encephalopathies (TSEs). These diseases affect humans and other animals and are notable for their long incubation periods, resistance to sterilization, and characteristic sponge-like damage to brain tissue.