Virus are obligate intracellular parasite which can reproduce only by invading and taking over other cells as they lack the cellular machinery for self reproduction.
Viroids are subviral and smallest known agents of infectious disease. The term viroid has recently been introduced by T.O. Diener (1971) to describe the causal agent of the potato spindle tuber disease. There are some 25 viroids which vary in nucleotide sequence.
Viroids are spread by plant propagation (cuttings and tubers) through seeds and by manual mishandling with contaminated implements
Virus vs Viroid
1. It is a nucleoprotein particle
2. Nucleic acid can be DNA or RNA
3. A protein covering of coat is present
4. Virus has a larger size.
5. Virus infects all types of organisms
1. It is an RNA particle
2. Viroid is formed of only RNA
3. A protein coat is absent
4. Viroid has a smaller size
*Virusoids are viroids located inside protein coat of true virus. RNA is linear or circular and non infections5. Viroid infects only plants