<unk> s are misfolded proteins with the ability to transmit their misfolded shape onto normal variants of the same protein. They characterize several fatal and transmissible neurodegenerative diseases in humans and many other animals. It is not known what causes the normal protein to misfold, but the abnormal three-dimensional structure is suspected of conferring infectious properties, collapsing nearby protein molecules into the same shape. The word<unk>  derives from "proteinaceous infectious particle". The hypothesized role of a protein as an infectious agent stands in contrast to all other known infectious agents such as viroids, viruses, bacteria, fungi, and parasites, all of which contain nucleic acids (DNA, RNA, or both).<unk>  isoforms of the<unk>  protein (PrP), whose specific function is uncertain, are hypothesized as the cause of transmissible spongiform encephalopathies (TSEs), including scrapie in sheep, chronic wasting disease (CWD) in deer, bovine spongiform encephalopathy (BSE) in cattle (commonly known as "mad cow disease") and Creutzfeldt–Jakob disease (CJD) in humans. All known<unk>  diseases in mammals affect the structure of the brain or other neural tissue; all are progressive, have no known effective treatment, and are always fatal. Until 2015, all known mammalian<unk>  diseases were considered to be caused by the<unk>  protein (PrP); however in 2015 multiple system atrophy (MSA) was hypothesized to be caused by a<unk>  form of alpha-synuclein.<unk> s form abnormal aggregates of proteins called amyloids, which accumulate in infected tissue and are associated with tissue damage and cell death. Amyloids are also responsible for several other neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease.<unk>  aggregates are stable, and this structural stability means that<unk> s are resistant to denaturation by chemical and physical agents: they cannot be destroyed by ordinary disinfection or cooking. This makes disposal and containment of these particles difficult. A<unk>  disease is a type of proteopathy, or disease of structurally abnormal proteins. In humans,<unk> s are believed to be the cause of Creutzfeldt–Jakob disease (CJD), its variant (vCJD), Gerstmann–Sträussler–Scheinker syndrome (GSS), fatal familial insomnia (FFI), and kuru. There is also evidence suggesting<unk> s may play a part in the process of Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS); these have been termed<unk> -like diseases. Several yeast proteins have also been identified as having<unk> ogenic properties, as well as a protein involved in modification of synapses during the formation of memories (see Eric Kandel#Molecular changes during learning).<unk>  replication is subject to epimutation and natural selection just as for other forms of replication, and their structure varies slightly between species.<unk> s are a type of intrinsically disordered protein, which change their conformation unless they are bound to a specific partner such as another protein. With a<unk> , two protein chains are stabilized if one binds to another in the same conformation. The probability of this happening is low, but once it does the combination of the two is very stable. Then more units can get added, making a sort of "fibril".
Prion
TRNA 
Lentivirus
Glycol nucleic acid
Genetically modified virus
Protein corona
Reporter virus particles
Microorganism