Characteristic features of Viruses

Viruses are unusual precisely because they show real, undeniable traits of both the living and the non-living world at once. On the 'living' side: they possess nucleic acid and protein, they are capable of mutation, they multiply (but only inside a living host cell), they can infect and cause disease, they show irritability, and they are host-specific (a given virus generally infects only a narrow range of hosts). On the 'non-living' side: they can be crystallised like a chemical compound, they show no metabolism of their own, they are completely inactive outside a host, they show no functional autonomy, and they carry no energy-producing enzym …