They require a host cell's metabolic machinery to reproduce. Click here to see examples of viruses. Additional Resources about viruses can be found by clicking here . Viruses have a protein coat called a capsid and a nucleic acid core of either RNA or DNA but neither both. Together the capsid and the nucleic acid core form the nucleocapsid.
Some viruses also have an envelope that surrounds the capsid.
Some enveloped viruses have spikes that project from the surface of the envelope. The influenza virus is an example of a virus that has spikes. The spikes allow the virus to attach to a host cell. A complete virus, consisting of at least a capsid and the nucleic acid core is known as a virion. Animal Viruses Click here to see my picture of the life cycle of an animal virus.