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|
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 OfSome 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.
HandwashingAntibacterial 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 eliminationThe 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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