Viral Infections
Viruses, the smallest of all parasites, can live, grow, and reproduce only within the living cells of a host organism, which may be any plant or animal, including human beings. The world is populated by many billions of viruses; most are harmless, but others are the sources of our most persistent and even deadly diseases -- everything from the common cold to AIDS.
In recent years, viral illnesses have gained new attention from researchers because viruses undergo genetic changes, or mutations, and new viral diseases keep cropping up. Researchers are especially concerned by the rapidly increasing global range and speed with which disease-bearing viruses can spread, due, at least in part, to wars, population density, human migration, deforestation, and air travel. Epidemiologists are attempting to establish an international warning system to spot newly evolving viral diseases quickly, so that they can be contained and treated when possible. These efforts were spurred by the 1995 outbreak in Zaire of the deadly Ebola virus, which causes widespread internal hemorrhaging.
There is mounting evidence that certain viruses may trigger a number of chronic diseases, including multiple sclerosis, Alzheimer's disease, and juvenile onset, or Type 1 diabetes. Viruses are also implicated in some malignancies. Cervical cancer, for example, is associated with the papilloma virus, which causes genital warts. Hepatitis C increases the risk of liver cancer, HIV appears to cause AIDS-related lymphoma in some AIDS patients and the herpes virus is implicated in Kaposi's sarcoma, another AIDS-related cancer. New insight into the possible viral origins of such diseases may ultimately lead to new vaccines and treatments.
In humans, viral infections are spread mainly by blood and respiratory and intestinal secretions. They may also be carried from one person to another by a biting insect. (Two examples: epidemics of dengue, or breakbone fever, have been traced to a mosquito originating in Hawaii; the Asian tiger mosquito, which entered the United States in 1986, is responsible for spreading the Eastern equine encephalitis virus.)
Although viral diseases differ greatly in severity and duration, they follow similar patterns of infection and replication. Once a virus invades the body, it enters its host's target cells. Some attach themselves to receptors on the cell's surface; others have proteins that allow them to penetrate the cell. Once inside, the virus sheds its coat or undergoes other changes that allow it to insert its genetic material into that of the host cell and then reproduce many times. Eventually, the new viruses escape from the host cell and travel through the body seeking new target cells. Depending upon the virus, they may travel through the bloodstream, the lymph system, or along nerve pathways until reaching their target organ.
Much remains to be learned about viral infections and their consequences. With so-called slow viruses, such as HIV, for example, there are no symptoms until months or years after, infection. Other viruses cause acute, but self-limiting illnesses; colds, flu, and measles are common examples. Still others, such as the varicella-zoster virus, which causes chickenpox and shingles, seem to disappear, but may re-emerge months or years later in somewhat different manifestations.
Fortunately, the body has many defenses against viruses, beginning with the skin. Those that enter through the respiratory or gastrointestinal tract face attack from the body's immune defenses as well as chemical and structural barriers. As soon as a virus enters the body, the immune system begins to make antibodies against it. This response does not always halt infection but it may result in immunity against future attacks. A bout of rubella or measles, for example, confers lifelong immunity against these diseases.
Diagnostic Studies and Procedures
Specific viral infections are usually diagnosed on the basis of the symptoms they produce. Epidemics of viral infections are tracked by state health authorities, laboratory reports, and the Centers for Disease Control and Prevention (CDC). Tissue and blood samples are examined for antibodies.
Medical Treatments
Many viral illnesses are short-lived and do not require treatment. Others, such as colds and flu, have no remedies, but there are medications that alleviate symptoms. Still others, such as AIDS, are incurable, but various treatments can slow the progress of the virus. Specific antiviral treatments include:
Immune globulins. These are substances derived from blood plasma taken from a large number of adults who have antibodies against the infecting virus. A few diseases that can be prevented by prompt administration of immune globulins following exposure to a specific virus are hepatitis A and B, chickenpox, and rabies.
Antiviral drugs. Although not as effective as antibiotics in treating bacterial infections, antiviral drugs can halt or lessen a viral infection if they are given early enough. Acyclovir (Zovirax), the first effective antiviral drug, is prescribed mostly for herpes infections, especially genital herpes and herpes encephalitis. It is also used to treat shingles in patients who have lowered immunity. Ribavirin (Virazole) is given as an inhalant for severe viral respiratory infections. Amantadine (Symmetrel) or rimantadine (Flumadine) is given to prevent or reduce the severity of influenza A, and vidarabine (Vira-A) to treat viral eye infections. There are also several new drugs used against HIV and its complications.
Interferon. This protective protein, which the body makes in response to a viral infection, is produced artificially by cloning techniques and is used to treat several diseases, including AIDS and other viral disorders.
Immunization. Effective virus vaccines include those for measles, mumps, rubella (German measles), influenza, hepatitis B, and rabies. A chickenpox vaccine is also now available.
Alternative Therapies
Some alternative therapies are directed toward boosting immunity and countering factors that increase susceptibility to viral infections. Others are used to alleviate symptoms.
Herbal Medicine. Garlic, taken fresh or in capsule form, is said to fight infections of all kinds. Herbs recommended to boost immunity include huang qu, available as a tincture or decoction, and purple coneflower (echinacea), taken as a powder or tincture.
Meditation and Visualization. Stress increases vulnerability to viral and other infections; meditation and other relaxation techniques can help offset its harmful effects on the immune system. Visualization, in which a person creates a mental image of the body fighting viruses, is also said to bolster immunity.
Nutrition Therapy. A healthy immune system requires a balanced diet that daily provides adequate protein, 5 to 9 servings of fresh fruits and vegetables, and 6 to 11 servings of starchy foods. Some nutritionists also recommend supplements of vitamins C and A and zinc to help prevent infection, but others feel these are unnecessary for people who consume a proper diet.
Self-Treatment
A healthful lifestyle -- a balanced diet, adequate sleep, regular exercise, abstinence from smoking, safe sex practices, and prudent use of alcohol -- is perhaps your best defense against viral illnesses. In addition, make sure you are immunized against the more common viral infections and that you have your scheduled booster shots. In planning a trip abroad, have any necessary immunizations before depamire.
Remember that most viral infections are self-limiting and some of your symptoms represent your body's way of fighting off an invading virus. For example, viruses are sensitive to heat; thus, a fever is actually pan of the healing process and is best left alone unless it rises to 104°F (40°C) in a child or 102°F (39°C) in an adult or lasts for more than two days. However, its necessary to drink extra fluids to prevent dehydration from a fever.
During a mild illness, eat small meals of light, easy-to-digest foods -- soups, gelatin, rice, toast. Switch to a liquid diet if diarrhea occurs. Try to stay in bed; extra sleep helps speed recovery.
Other Causes of Infections
Bacteria, fungi, and a variety of parasites can produce infections.