Viruses are small protein-enclosed particles that contain their own genetic material (either DNA or RNA) but can’t reproduce outside of a host cell. They are obligate parasites that need to infect host cells to replicate and they infect a narrow range of hosts. For example, the viruses that cause most common colds in humans (rhinoviruses) are only able to infect cells in the upper respiratory tract of humans. After infection, the host cells die when new viruses burst from them.
Recently, biologists have come to realize that viruses play important roles in regulating and modifying most aspects of important biological processes. For example, we now know that viruses are incredibly abundant in the oceans. Measurements suggest that a milliliter of sea water contains ~107 viruses and these viruses infect every type of organism in the sea from bacteria to whales.
Recently, a virus has been discovered (Chaetoceros salsugineum nuclear inclusion virus (CsNIV)) that infects Chaetoceros salsugineum , a species of diatom. Diatoms of the genus Chaetoceros are abundant components of marine plankton communities. They are known to contribute to global atmospheric oxygen and carbon dioxide levels and are a major source of nutrition for organisms higher on the food chain. Several species of Chaetoceros are known to parasitize fish by clogging their gills.