Final answer:
Prions are pathogenic proteins that unfold normal proteins, leading to fatal brain diseases. Prions spread to humans primarily through consumption of infected food or contamination, and are highly resistant to traditional deactivation methods.
Step-by-step explanation:
Prions are infectious, misfolded proteins that lack DNA or RNA and are known to cause several fatal brain disorders, such as mad cow disease and Creutzfeldt-Jakob disease. Prions propagate by converting normal prion proteins into the abnormal, disease-causing form. The presence of these prions can lead to a build-up in brain tissue, resulting in damage and the characteristic spongy appearance.
Unlike viroids, which are single-stranded RNA molecules that infect plants, prions affect animals and humans and do not require a capsid or envelope. Prions are exceptionally resistant to destruction by heat, chemicals, and radiation, making them very difficult to control. The spread of prions to humans can occur through consuming infected meat or through contaminated medical instruments.