01 October 2005

What Are Viruses?

Bird flu, AIDS, Ebola, Smallpox and the Common Cold -- These are caused by "viruses". These little units of genetic information are a major part of life on Earth, whether in the News or just in the nose.

Viruses and bacteriophage are small packets of DNA or RNA that infect the cells of other organisms (they attack eukaryotes in the case of viruses, bacteria in the case of phage). They don't possess the biochemical machinery to reproduce themselves, so they hijack that of their host cells. Whether viruses are living organisms is a question of semantics (it depends on what you mean by "living").

Diverse organisms are lumped into this group, as some have their genetic information in the form of DNA, some as RNA; in some the genetic material is single-stranded and in others double-stranded; and it can be either positive or negative in its orientation. The genetic material generally has an enclosing capsule made of protein subunits, but the capsule may also incorporate lipids or glycoproteins.

Electronmicrograph of Herpes virus particles
Viruses are small, from about 20 nanometers to about 400 nanometers in size. (A bacterial cell is generally in the range of 0.5 to 5.0 micrometers in size. A micrometer is one thousand times bigger than a nanometer, so bacteria are hundreds of times larger than viruses.) photo of cold sore on lip, from http://medicineworld.org/cancer/lead/6-2006/why-that-cold-sore-keep-coming-back.html(The wavelength of visible light is several hundred nanometers, so viruses can only be seen using the very short wavelengths of electron microscopes. Their effects can be easy to see, however. A "cold sore" is a region of dead tissue where cells have been attacked, killed, and burst by the virus Herpes simplex.)

We don't know how viruses evolved, but the current thinking is that they developed from their host organisms. They are not primitive, but derived. Some may be extremely reduced forms of single-celled organisms.

Viroids and virusoids (single naked RNA strands that infect plants) and prions (infectious self-replicating proteins) are other simple infections systems, but are not technically viruses.

Viruses You May Have Heard Of

Some well-known viruses include:
  • Common Cold -- Several hundred viruses can cause the upper respiratory problems we call "colds", including rhinoviruses, coronaviruses, and some echoviruses, paramyxoviruses and coxsackieviruses.
  • Smallpox -- caused by the Variola virus. Currently extinct in the wild, but stocks still exist in laboratories.
  • Influenza -- caused by an RNA virus.
  • HIV (Human Immunodeficiency Virus) -- a retrovirus that causes AIDS by infecting cells of the immune system.
  • Herpes virus -- which causes cold sores and genital herpes
  • Varicella-zoster virus -- the cause of chickenpox, and which when reactivated in the elderly causes the painful skin ailment shingles.
  • Human papillomavirus -- Some strains cause common skin warts, while others are associated with sexually transmitted diseases. The sexually transmitted strains may cause no harm, cause genital warts, or even cause cervical cancer, which fortunately can be detected early by the Pap smear test.
  • Filoviridae -- such as the one that causes Ebola haemorrhagic fever.
  • Bird flu -- or avian influenza, caused by members of the Orthomyxoviridae family of negative-stranded RNA viruses. Some strains can infect humans, and there is serious concern that a virulent, highly transmissible strain could develop by mutation in the large wild reservoir of the virus in birds and other domestic animals.
  • The SARS coronavirus -- cause of severe acute respiratory syndrome (SARS). A single-strand positive-sense RNA virus.

How Can Viruses Be Controlled?

Viruses cannot be killed by antibiotics. Antibiotics kill or stop the growth of bacteria, not viruses. Using antibiotics to try to control viral diseases like colds and flu just hastens the day those antibiotics will be useless against dangerous bacteria, because exposing populations of bacteria to antibiotics gives them a chance to evolve defenses against the drugs.

Recently some anti-viral drugs have been developed. They are used to try to keep HIV infections under control to prevent development of AIDS. There are other antiviral drugs useful in controlling influenza. Some of these anti-influenza drugs may be helpful against "bird flu" (see this article), but because of stockpiling there is often not enough available. Also, recent results indicate that viruses may already be developing resistance to these drugs.

Virus-caused diseases can be prevented by vaccination.

The spread of viruses that can cause disease can be minimized by barriers, such as gloves, condoms, and masks.

Handwashing

Antibacterial soaps don't kill viruses, but just washing with any soap will remove or inactivate many of the viruses on your hands. Alcohol-based hand cleaning systems also reduce viral load.

Vector avoidance or elimination

The spread of viruses can be reduced by controlling the vectors that spread the viruses, e.g. the mosquitoes that spread west nile virus.

Viruses are everywhere. They are constantly mutating, so they are always threatening to become a major public health catastrophe and news story. Bird flu, for example, currently mainly affects birds, though it has caused dozens of human deaths. A virulence mutation could turn this highly-contagious virus into a deadly cause of epidemics.


Post on Epidemics here.

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3 comments:

njmandela said...

There is some amazing news with regards to cold sores. Professor Bloom of Florida University has discovered a novel way to inhibit the virus. Check it out:

Cold Sore News

-NJ

Anonymous said...

Is there a difference between the flu and a cold? I've always been told that there was; the flu being the more severe one. I even read that the viruses were different; the influenza virus and the corona virus. But a while back I was at the doctor's and he diagnosed my symptoms as the flu. I said that I thought it was just a cold. He then said that they were the same thing. So what is it?

D. Wheat said...

You're both right! The "common cold" is a viral upper-respiratory tract infection that lasts a few days or a week or so, causing sneezing, runny nose, etc. It can be caused by any of a number of viruses.
"Flu" is a more severe disease, which also has fever, muscle aches, etc., and usually lasts longer. It is caused by a different set of viruses.
(There is also "stomach flu", gastroenteritis, which is caused by any of a range of viurses, bacteria, or parasites and involves vomiting and/or diarrhoea.)
Your doctor is right in a sense, however, since colds and flu are both upper respiratory viral infections, therapy is usually about the same. Rest, drink lots of fluids, take asperin or acetomenophen for fever or aches. You will get better in a few days or a week.
If you get a virulent form of influenze, like avian influenza, you have trouble breathing, and you are elderly, you will probably be hospitalized and given anti-viral drugs.