Viruses exist in two phases - an extracellular phase where they possess few enzymes and protect their genome, and an intracellular phase where they induce host cells to synthesize viral components. They are cultivated using techniques like growing them in embryonated eggs, animal cell monolayers, and bacterial lawns. Viruses have a nucleocapsid core containing genetic material surrounded by a protein capsid. They vary greatly in size and structure, with capsids that are isocahedral, helical, enveloped, or more complex. Their genetic material can be single or double-stranded DNA or RNA.