Prion disease can also be referred to as transmissible spongiform encephalopathy, and is abrare advanced neurodegenerative disorders of brain. The brain tissue degenerates and healthy tissue is replaced by clusters of tiny liquid- filled, thin-walled cysts which makes the brain looks like sponge.
Prion diseases occur when "infectious proteins" called prions misfold and then recruit normal proteins to do likewise. The misshapen proteins clump together, eventually killing brain cells. These diseases at present can be diagnosed definitively only by examining a brain after death. Misshapen molecules have been linked to Creutzfeldt-Jakob disease and more common, non-infectious brain diseases such as Alzheimer's, Parkinson's, Huntington's and Lou Gehrig's.
If prions truly are the culprits behind these diseases, their existence flies in the face of the conventional wisdom that only organisms with a genome (such as viruses or bacteria) can spread disease or perpetuate themselves in living cells. Not all scientists accept the "prion hypothesis". Although plenty of evidence hints that prions act alone to cause disease, no one has yet been able to conclusively prove this theory.