Viruses are infective nucleoprotein particles i.e., have nucleic acid as their genetic material and protein coat. They can produce their own. copies inside the host cell. On the basis of number of strands present in nucleic acid the viruses are of four types .
(i) Single stranded DNA virus-bacteriophage ϕ X 174, coliphage S13, bacteriophage M13.
(ii) Double stranded DNA virus-adenovirus, herpes virus, smallpox, polyoma virus, SV-40, cauliflower mosaic virus, dahlia mosaic virus.
(iii) Single stranded RNA virus? TMV, influenza, rubeola HIV, polio, rabies, RSV etc. (iv) Double stranded RNA virus? wound tumour virus, rice dwarf virus, reovirus etc.