Sunday, January 4, 2009 3:01 PM
Introduction to virusesViruses
A virus is a biological agent that reproduces inside the cells of living hosts.
When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate.
Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell.
All viruses have genes made from either DNA or RNA, long molecules that carry the genetic information; all have a protein coat that protects these genes; and some have an envelope of fat that surrounds them when they are not within a cell.
Viruses vary in shape from the simple helical and icosahedral to more complex structures.
The origins of viruses is unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
Viral infections often cause disease in humans and animals, however they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus.
Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections.
Vaccines that produce lifelong immunity can prevent some viral infections.
Done by: Nicolina Ng