Viroids are sub-viral agents as infectious RNA particles, without a protein coat.
Virus:- It is a small infectious agent that replicates only inside the living cells of other organisms. It is an obligate intracellular parasite which can reproduce only by invading and taking over other cells as they lack the cellular machinery for self-reproduction.
Viroids:- These are the smallest infectious pathogens known. These are subviral, smallest known agents of infectious disease. The term viroid was coined by T.O. Diener (1971) to describe the agent of the potato spindle tuber disease. There are some 25 viroids which vary in nucleotide sequence. These are spread by plant propagation through seeds and by manual mishandling with contaminated implements.