Concepts Viruses are simple,acellular entities consisting of one or more molecules of either DNA or RNA enclosed in a coat of protein. They are reproduced only within living cells and are obligately intracellular parasites .The nucleic acid strands can be linear,closed cycle,or able to assume either shape. .Viruses are classified on the bases of their nucleic acid's characteristics,capsid symmetry,the presence or absence of an envelop,their host and other properties.Concepts •Viruses are simple, acellular entities consisting of one or more molecules of either DNA or RNA enclosed in a coat of protein. They are reproduced only within living cells and are obligately intracellular parasites •The nucleic acid strands can be linear, closed cycle, or able to assume either shape. •Viruses are classified on the bases of their nucleic acid’s characteristics, capsid symmetry, the presence or absence of an envelop, their host and other properties