Prion concentration in birth fluids does not alter the infectivity of the prions. The definition of a prion-like domain arises from the study of fungal prions. Humans can be infected with many types of pathogens including prions, viruses, bacteria, and fungi. Kuru is a rare, orally transmitted, incurable neurodegenerative disorder that is caused by prions. The concentration of the prions is uncertain, but is not directly proportional to infectivity. For their mechanistic similarity to mammalian prions, they were termed yeast prions. Normal sterilization procedures such as boiling or irradiating materials fail to render prions non-infective. It is likely to be highly effective at killing pathogens, including prions. It is caused by proteins called prions. Fungal prions have provided a model for the understanding of disease-forming mammalian prions. It also reduces the risks of contaminants such as host viruses, toxins and prions. An infected deer's saliva is able to spread the CWD prions. They are classified as viruses, viroids, prions, or satellites. Deletion of the HSP104 gene results in cells that are unable to propagate certain prions. Hsp104, the Hsp100 of Saccharomyces cerevisiae, is essential for the propagation of many yeast prions.