Of all the infectious agents, viruses are the most unique and the above image gives a great outline of their diversity. Responsible for many of the most common and serious human diseases, they remarkably remain metabolically inert and lack the ability to replicate on their own.
Structurally, viruses primarily consist of genetic material in the form of DNA or RNA which is protected by a highly ordered cage of proteins, known as the capsid. Some viruses also contain an additional outer envelope or membrane consisting of lipids, or glycoproteins.
The outer surface of the virus enables it to make contact with the outer membrane of a host cell and facilitates its uptake into the cytoplasm. Once there, the virus hijacks the host cell’s machinery enable its replication and the production of viral proteins. Newly replicated viruses are then either released via rupture of the host cell, or the viral genetic material is incorporated into and replicated with the host cell’s genome.