Human Viral Diseases Are Common

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Many common childhood diseases such as measles, mumps and chickenpox are caused by viruses, as are the common cold and flu. More dangerous viral diseases include polio, smallpox, herpes, Lassa fever, Ebola and AIDS. Do viruses ever do anything useful? Yes; infection by a mild virus can provide resistance against a related but more dangerous virus (see Ch. 17). Viruses may carry genes from one host organism to another, in a process known as transduction (see Ch. 18), and have thereby played a major role in molecular evolution (see Ch. 20). The ability of viruses to carry genes between organisms may be put to good use by genetic engineers. All the same, about the best that can be said for the natural role of viruses is that most of them do relatively little damage and only a few cause highly virulent diseases.

Viral diseases usually cannot be cured once they have been caught. Either the victim's body fights off the infection or it does not, although some antiviral drugs can help the host in the fight. However, viral diseases can often be prevented by immunization, if a potential victim is vaccinated before catching the virus. In this case, the invading virus will be killed by the immune system, which has been put on alert by the vaccine, and the disease will be prevented.

Antibiotics are of no use against viruses; they only kill bacteria. So why do doctors often prescribe antibiotics for viral diseases like flu or colds? There are two main reasons. The valid reason is that giving antibiotics may help combat secondary or opportunistic infections caused by bacteria, especially in virally-infected patients who are in poor health. However, massive over-prescription of antibiotics occurs because many patients would be upset if faced with the truth. They would rather be given medicine, even if it is of no use, than face the fact that there is no cure. This abuse has in turn contributed to the spread of antibiotic resistance among many infectious bacteria (see Ch. 16)—thus creating a major health problem.

immunization Process of preparing the immune system for future infection by treating the patient with weak or killed versions of an infectious agent vaccination Artificial induction of the immune response by injecting foreign proteins or other antigens

A Variety of Subcellular Genetic Entities Exist 49

FIGURE 2.30 The Variety of Subcellular Genetic Elements—"Gene Creatures"

These structures possess some of the characteristics of life. However, they use their host's machinery to replicate. The plasmid and the viroid lack a protein shell. The transposon is merely a segment of DNA (yellow) with special ends (blue) inserted into another DNA molecule.

Dna virus

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A Variety of Subcellular Genetic Entities Exist 49

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