What is the smallest infectious pathogen known, consisting solely of a short strand of circular, single-stranded RNA?

The smallest infectious pathogen known, consisting solely of a short strand of circular, single-stranded RNA, is a viroid. Viroids are unique because they lack a protein coat, which differentiates them from viruses. They are significantly smaller than both typical viruses and bacteria, often just a few hundred nucleotides in length. Viroids primarily infect plants and can cause various diseases, disrupting the normal physiological functions of their host.

In contrast, viruses are composed of genetic material (either RNA or DNA) encased in a protein coat and often require a host cell to replicate. Bacteria are single-celled organisms with a more complex structure, including a cell wall and cellular machinery for reproduction. Prions are misfolded proteins that induce other normally folded proteins to misfold, leading to neurodegenerative diseases; they do not contain RNA or DNA.

Viroids illustrate an interesting aspect of molecular biology, demonstrating that nucleic acids alone can be pathogenic, highlighting the diverse mechanisms through which infectious agents can operate.