Also, within the stroma DNA and ribosomes similar to those in bacteria are present. Living archaeons include the methanoges, the halophiles and the thermophiles. All other prokaryotes are in the domain Bacteria. Viruses Viruses, like bacteria, are responsible for many diseases. Viruses are known to be responsible for AIDS, polio, influenza, cold sores, measles, and a few types of cancer.
All viruses are obligatory intracellular parasites.
They can not reproduce unless present within a host cell, which depending on the specific virus may be a plant, animal or bacterial cell. Outside of a living cell, the virus exists as a particle, or virion, which is little more than a macromolecular package. The genetic material of the virion is surrounded by a protein capsule. Each virus has on its surface a protein that is able to bind to a particular surface component of its host cell. Viroids Viroid is an infectious agent consisting of a small circular RNA molecule that totally lacks a protein coat.