Viruses are the smallest infectious organisms, and they are so tiny that millions of them could fit inside a single human cell. Viruses are only capable of reproduction inside a living cell, called a host cell, that they invade. A virus consists of little more than a single or double strand of genetic material surrounded by a protein shell. However, some types of virus also have a protective outer envelope.
Changes in viruses
The immune system recognizes viruses by the proteins that are on their surfaces (antigens). When viruses reproduce, the antigens on the new viruses may become slightly different to prevent the immune system from recognizing the virus. This is known as antigenic drift. Much larger changes (antigenic shifts) may result in epidemics.
How viruses reproduce
To survive, viruses must reproduce inside living cells. The genetic material from an infecting virus takes over the functions of the host cell to make millions of new virus particles. The new viruses leave the host cell by bursting out of the cell or by budding out from the cell surface.
Proteins on the virus attach to specific receptors on the surface of a host cell. The virus may enter the cell by being engulfed by the cell membrane or by fusing into the cell membrane.
When inside the cell, the virus sheds its protein shell. The genetic material of the virus reproduces, using substances from inside the cell.
Each copy of the genetic material programmes the formation of a new protein shell. Once the shells have formed, the new viruses are complete.
The viruses leave the cell either by suddenly rupturing the cell membrane, which destroys the host cell, or by slowly budding out from the surface of the cell membrane.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
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