I. Introduction

• Viroids are circular RNA molecules that infect plants, causing abnormal growth and stunted development
• Prions are infectious proteins that cause degenerative brain diseases in various animal species, including humans
• These smallest of infectious agents pose significant challenges in terms of understanding and preventing their spread

II. Viroids

• Viroids are circular RNA molecules that are a few hundred nucleotides long
• They do not encode proteins but can replicate in host plant cells using host cell enzymes
• Viroids cause errors in the regulatory systems that control plant growth, leading to disease
• An example of a viroid disease is cadang-cadang, which has killed over 10 million coconut palms in the Philippines
• Viroids serve as an example of how a single molecule can be an infectious agent

III. Prions

• Prions are infectious proteins that cause degenerative brain diseases in various animal species, including humans
• Examples of prion diseases include scrapie in sheep, mad cow disease in cattle, and Creutzfeldt-Jakob disease in humans
• Prions can be transmitted through food, such as beef from cattle with mad cow disease
• Kuru is another human disease caused by prions and was identified among the South Fore natives of New Guinea
• Prions act slowly, with an incubation period of at least ten years, and are virtually indestructible
• There is currently no cure for prion diseases, and the only hope for developing effective treatments lies in understanding the process of infection

IV. Prion Mechanism of Infection

• Prions are a misfolded form of a protein normally present in brain cells
• They convert normal protein molecules to misfolded prion versions through aggregation
• Prion aggregation interferes with normal cellular functions and causes disease symptoms
• The leading model for prions was proposed by Stanley Prusiner and has since been widely accepted
• Prusiner was awarded the Nobel Prize in 1997 for his work on prions and has recently proposed that prions may also be involved in neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease

V. Conclusion

• Viroids and prions are the smallest of infectious agents and pose significant challenges in terms of understanding and preventing their spread
• While much is still unknown about these agents, ongoing research continues to provide new insights into their mechanisms of infection and potential treatments.