Here's why:

* Prions are infectious agents composed solely of protein. They lack any genetic material like DNA or RNA.

* They cause a variety of diseases in mammals, including humans, known as prion diseases or transmissible spongiform encephalopathies (TSEs). These diseases are characterized by progressive neurodegeneration.

* Prions work by misfolding normal proteins in the brain, causing them to become infectious as well. This leads to a chain reaction of misfolding, resulting in the formation of amyloid plaques that damage brain tissue.

Famous examples of prion diseases include:

* Bovine spongiform encephalopathy (BSE) or "mad cow disease"

* Scrapie in sheep

* Creutzfeldt-Jakob disease (CJD) in humans

Prions are fascinating and somewhat frightening because they demonstrate that infectious agents don't necessarily need genetic material to spread and cause harm.