Viruses

A virus is a submicroscopic infectious particle composed of a protein coat and a nucleic acid core. Viruses are similar in size to a large protein macromolecule, generally smaller than 200 nm in diameter. Viruses, like cells, carry genetic information encoded in their nucleic acid, and can undergo mutations and reproduce; however, they cannot carry out metabolism, and thus are not considered alive. Viruses are classified by the .type of nucleic acid they contain, and the shape of their protein at capsule
Retroviruses
Retroviruses use RNA instead of DNA as their nucleic acid core. They also contain the enzyme reverse tranb,c.riptase, which will detranscribe the RNA sequence into a DNA strand. Once the ret~6viral RNA and reverse transcriptase are inside the host cell the enzyme reverses trarlscription by making a single stranded DNA from the retroviral RNA. Viral DNA can be integrated into the host DNA. It remains in the genome and is replicated whenever the host DNA replicates. If viral DNA is transcribed, new viruses are produced by biosynthesis, maturation, and release by budding. Retroviruses include HIV and also cause certain forms of cancer.
Viruses and Diseases
Viruses cause a vahety of diseases among all groups of living things. Viral plant diseases can be controlled solely by burning those plants that show symptoms of disease. Viral diseases in humans are controlled by preventing transmission, administering vaccines, and only recently by the administration of antiviral drugs. Virally caused human diseases include the flu, common cold, herpes, measles, chicken pox, small pox, and encephalitis. Antibiotics are not effective against viruses. Vaccination offers protection for uninfected individuals. Frequent hand washing and condom use may help prevent transmission. Vaccines are substances that stimulate an immuneresponse wi,thout causing the illness. Commonly used virus vaccines include polio, measles, and mumps.
Antibiotics do not cure viral infections because viruses use enzymes produced by the host cell, rather than produce not their own. A few antiviral drugs are available that interfere with viral replication without interfering with host metabolism in cells free of the virus. Antivirals include acyclovir for herpes and AZT for AIDS. Despite recent successes with antiviral drugs, vaccination and the prevention of exposure remain the most effective ways to deal with viral infections.
Emergent Viruses
Viruses are usually quite specific as to their hosts and even to the types of cells they infect in a multicellular host. Recently, some viruses appear to have shifted their host: HIV, hantavirus, and ebola appear to be either viruses shifting to a new (human) host or else viruses whose existence and effects are just now being realized by scientists and the general public.
Viroids and Prions
Viruses would apperar to be the simplest form of infectious particle. The discovery of viroids, nucleic acid without a protein capsule, and prions, infectious proteins, subtracts another level of complexity. Both viroids and prions can cause diseases, the most famous of which is mad cow disease (causes by a prion).