Virus is an agent that causes an infectious disease.
Virion is a complete virus fragment.
Viroid is an agent that causes an infection to plants.
Differences:
1. Virus has a protein coat that surrounds the RNA or DNA core but has no semipermeable membrane.
2. Virion has RNA and DNA core coated with protein and external envelops indicating as an extracellular infectious form of a virus.
3. Viroid is kinda similar to a virus. Thus, it only has single strand of RNA and has no protein coat.
