العَوَامِلُ دُونَ الفَيْرُوسِيَّةِ (Subviral Agents)

Viroids are circular, single-stranded RNA molecules that lack a protein capsid, unlike conventional viruses. They consist only of RNA without encoding any proteins. Instead, their structured RNA functions as small interfering RNAs that silence plant genes, causing disease through gene interference rather than protein production.

Prions are misfolded proteins (PrPSc) that induce normal cellular proteins (PrPC) to misfold into the same aberrant shape. These aggregates accumulate in neural tissue, forming amyloid fibrils that damage neurons and cause transmissible spongiform encephalopathies. This progressive brain degeneration creates vacuoles and neuronal loss characteristic of prion diseases.

Satellites possess either RNA or DNA enclosed in a capsid but lack genetic information for independent replication. They require co-infection with a helper virus to replicate and be packaged. Hepatitis D virus exemplifies this dependency, relying on Hepatitis B virus for replication while exacerbating liver disease in co-infected individuals.

Viroids rely entirely on host cell machinery for replication, using the plant's RNA polymerase through a rolling-circle mechanism. Their cleavage into functional units occurs via ribozyme activity, which is catalytic RNA within the viroid itself. This self-cleaving process releases individual RNA units without requiring viral enzymes.

Prion diseases can be inherited genetically, arise spontaneously, or be acquired through exposure to contaminated food, surgical instruments, or tissue transplants. Creutzfeldt-Jakob disease in humans exemplifies acquired transmission, while mad cow disease in cattle and scrapie in sheep demonstrate how prion infections spread through infected meat and animal products.

Viroids lack a capsid and do not encode proteins, whereas viruses with RNA genomes possess a protein coat and encode viral proteins for replication. Viroids function as small interfering RNAs that interfere with host gene expression, while viruses with RNA genomes use their genetic material to produce proteins necessary for viral replication and infection.

Helper viruses provide essential replication machinery and packaging proteins that satellites cannot produce independently. Satellites modify disease symptoms caused by the helper virus while depending on it for their own replication and encapsidation. This obligate relationship makes satellites unable to establish infection without co-infection with their specific helper virus.