All About Viruses
Viruses are familiar from the common diseases they cause: colds, flu, and chickenpox for instance. But what are they, and how do they cause sickness?
A virus is a tiny, infectious particle made up of an outer coat wrapped around a strand of DNA or RNA. DNA and RNA are chains of genetic material that contain instructions for the virus to reproduce. But, viruses cannot reproduce without a host cell. The virus identifies a host cell and injects its genetic material into the host cell. This allows the virus to take over the host cell's functions. The host cells is then directed to multiply and make more of the virus.
How a virus infects a cell
Viruses can stay in the body area or organ they first infect, or they can spread. Viruses that cause hepatitis, for example, infect the liver and stay there. The measles virus and varicella-zoster virus enter through the respiratory tract and spread to lymph nodes, skin and other organs. Viral infections can damage body tissues in several ways. They can interfere with the normal processes of the host cell, kill the host cell by exploding out of it, or trigger the immune system's response.
In people with a healthy immune system, viruses can produce infections that last from 7 to 14 days. Some viruses, however, can cause chronic infections. Others lie undetected in the body and cause symptoms at a later time. This is called a latent infection.
In a chronic infection, the virus reproduces and causes effects for an extended time, perhaps for a person's entire life. Hepatitis B and C viruses cause a chronic infection.
In a latent viral infection, the virus's DNA or RNA rests harmlessly in the host cells and does not reproduce. If the virus is eventually activated, it begins to reproduce and damage body tissues. Varicella viruses are examples of viruses that cause latent infections. The varicella-zoster virus remains in the body after causing the initial infection known as chicken pox. After the initial infection, it enters the nervous system where it remains dormant, not reproducing and not causing tissue damage. If it is re-activated, it travels through nerves to the skin, where it causes the blister-like lesions of shingles. The lesions appear along the route that the affected nerve follows underneath the skin. The virus then returns to its dormant state.
Outside the body, viruses can be killed by detergents, bleach, organic solvents such as ether or chloroform, and ultraviolet (UV) light.
Inside the body, the immune system makes antibodies against specific viruses. Antibodies are made when the immune system first encounters a virus. The body builds an antibody specially designed to prevent that particular virus from attaching to new cells. Once an antibody is made for a specific virus, the immune system usually continues to make it, but in much smaller quantities, even if there is no current viral attack. If the immune system encounters that virus again, its response will be faster because it does not have to build a new antibody. It simply makes more of the ones it already has. This is called immunity.
You can develop immunity to fight a future viral infection in two ways. You can catch the virus or get a vaccination. Vaccines are made from a killed or inactivated form of the virus or from harmless parts of a viral cover grown in a laboratory. These substances contain just enough of the virus to trigger the immune system to build an antibody, but not enough to cause infection. Vaccines exist for these viruses: chicken pox, shingles, measles, mumps, rubella, hepatitis A, hepatitis B, yellow fever, human papillomavirus, rabies, influenza, polio, Japanese encephalitis and rotavirus.
Another of the body?s natural defenses against viral infections is a family of proteins called interferons. Interferons also fight bacterial infections and tumors. They do not kill viruses, but they activate other immune responses in the body, including processes in host cells that stop the virus's activity. Interferons can also be made commercially and injected into the body to boost the immune system response.
Once a virus is inside a host cell, it is difficult to kill or damage it without killing or damaging the cell. Because of this, scientists have developed drugs that interfere with a virus's functions rather than killing it outright. Antiviral drugs have been developed that prevent the virus from attaching to a host cell, entering the cell, reproducing within a cell, or releasing newly formed viruses. The drugs amantadine and rimantadine, for example, work by preventing the virus from entering the cell; the drug acyclovir blocks viral reproduction within the cell. Two newer drugs for the treatment of influenza, zanamivir and oseltamivir, block the release of newly formed viruses from the host cells, preventing their spread to other host cells. Protease inhibitors, used in treatment of HIV, work by blocking an enzyme the HIV virus uses to make copies of itself.
Antibiotics, which are prescribed for bacterial infections, don't work against viruses. This is because antibiotics are designed to interfere with biochemical reactions bacteria need to survive. Viruses don't have these same biochemical reactions.