virus

the smallest microbe that is completely parasitic because it is capable only of replication with in the CELLS of its host. Viruses infect animals, plants and microorganisms. Viruses are classified according to their nucleic acids and can contain double or single-stranded DNA or RNA . In an INFECTION , the virus binds to the host cells and then penetrates to release the viral DNA or RNA , which controls the cell's METABOLISM . It then replicates itself and forms new viruses. Viruses cause many diseases, including INFLUENZA (single-stranded RNA ), HERPES (doublestranded DNA ), AIDS (a RETROVIRUS , single-stranded RNA ) and also MUMPS , CHICKENPOX and POLIOMYELITIS .