Identification of Viruses and Diagnosis of Viral Disease
BACKGROUND

“…as far as the rabies virus is concerned, a dog is just another way of making a virus”
V.S. Ramachandran and Sandra Blakeslee (1)

Have you ever wondered, while in the midst of a horrible bout of the flu or the common cold, about the purpose of a virus? It may surprise you to know that biologists do not consider viruses to be living things. Officially, they are “obligate intracellular parasites”, meaning they have no choice but to exist below the level of living cells. In other words, viruses are inactive (“inert”) unless they enter a living host cell. Once inside, they become active, forcing the host cell to use its machinery to make more viral particles, each of which can go on to repeat the viral replication cycle. This causes the varying degrees of misery seen in the host. From the viral point of view, our misery is just part of its replication strategy. Harmful viruses tend to replicate rapidly, “hoping” to spread to another host before having to leave the sinking ship. Milder viruses replicate more slowly, hiding out under the radar of the body’s immune system. Either way, the host is just a way of making a virus.

So what then is a virus? Since they operate within living cells, they are necessarily very small, only 20 to 14,000 nm in length, viewable only with an electron microscope. Viral structure is elegant in its simplicity: essentially viral genes in the form of either a DNA or RNA molecule in a protein coat called a capsid. Depending on the virus, its particles may also include viral enzymes, which help catalyze the viral replication process. Capsids tend to consist of regularly arranged protein clusters called capsomers. The resulting structure can be likened to a multi-faceted diamond, which may or may not be covered with membrane “borrowed” from the host cell. Viruses are classified according to their RNA or DNA, and their host range; it is the capsid structure that determines which specific cells can be infected. For example, viruses called bacteriophages can only attack specific bacteria. During infection, the viral genes act as a “copy-me” message, similar to a digital chain letter. Infection is a cascade of viral gene expression resulting in highly concentrated viral component parts within the cell. Because viral structure makes so much sense, energetically speaking, viral parts spontaneously self-assemble into fully infectious particles (“virions”) as soon as they reach a critical threshold concentration within the host cell.

Definitions:

Additional sources:
There are many free websites that offer excellent information about viruses. Here are a few of our favorites:

Timeline of virus-related discoveries:
1. http://www.medicine.wustl.edu/~virology/index.htm

Viral images:
1. http://www.uct.ac.za/depts/mmi/stannard/linda.html
2. http://www.virology.net/Big_Virology/BVHomePage.html
3. http://bocklabs.wisc.edu/virusviztop.html
4. http://mmtsb.Scipps.edu/viper/viper.html

General information and viral taxonomy:
1. http://life.anu.edu.au/viruses/welcome.htm
2. http://www.accessexcellence.org/AB/GG/#Anchor-Viruses-44867
3. http://www.iah.bbscr.ac.uk/virus
4. http://www.tulane.edu/~dmsander/garryfavweb.htm

Animations and movies of viral infections:
1. http://www.asmusa.org/pcsrc/mlp/movies/t4phage.mov
2. http://www.imagesite.com
3. http://www.hhmi.org/lectures/biointeractive/animations/infection/inf middle frames.htm
4. http://www.cellsalive.com/movs/phage.mov
5. http://www.cellsalive.com/phage.htm

Bacteriophages:
1. http://www.mansfield.ohio-state.edu/~sabedon

Viral infectious disease:
1. http://www.nationalatlas.gov/virusmap.html
2. http://www.cdc.gov/ncidod/EID/eid.htm
3. http://www.outbreak.org/cgi-unreg/dynaserve.exe/outbreaks.html
4. http://info.med.yale.edu/EIINet/welcome.html
5. http://www.fluwatch.com/index2.htm
6. http://www.avma.org

Relevance to Academia and Industry:
Viruses are medically important because they can cause disease in humans, economically important because they can cause disease in agriculturally significant organisms, and biotechnologically important because they can be used as gene-delivery vehicles.