A virus is a small infectious organism–much smaller than a fungus or bacterium–that needs a living cell in order to reproduce. The virus attaches to a cell, often a specific type of cell. Once inside a cell, the virus releases its DNA or RNA (which contains the information needed to create new virus particles) and takes control of some aspects of the cell's metabolism. The components of the virus are then manufactured inside the cell and must be properly assembled for the virus to be released and remain infectious.
What happens to the cell depends on the type of virus. Some viruses kill the cells they infect. Others alter the cell function so that the cell loses control over normal cell division and becomes cancerous. Some viruses incorporate a part or all of their genetic information into the host cell DNA, but they remain silent (or latent) until the cell is disturbed in a way that permits the virus to emerge again.
Most viruses have a preferred host. Some, such as the influenza virus, can infect humans and a variety of other animals. However, some strains of influenza have adapted in a way that allows them to infect one species of animal more efficiently than others. Most viruses commonly found in people are transmitted from person to person. Some viruses, such as the rabies virus or encephalitis viruses, infect animals primarily and humans only occasionally.
The body has a number of specific and nonspecific defenses against viruses. Physical barriers, such as the skin and mucous membranes, discourage easy entry. Infected cells also make interferon(s), a family of glycoproteins that can make noninfected cells more resistant to infection by many viruses.
If a virus does enter the body, various types of white blood cells, such as lymphocytes, are able to attack and destroy infected cells. The two main types of lymphocytes are B lymphocytes and T lymphocytes. When exposed to a viral attack, T lymphocytes increase in number and mature into either helper cells, which aid antibody-producing B lymphocytes, or cytotoxie (killer) cells, which can attack cells infected by a specific virus. T lymphocytes also produce chemicals (called cytokines) that speed this maturation process. The cytokines from helper cells can help B lymphocytes and their derivatives, the plasma cells, to produce antibodies that target specific viruses and make them noninfectious before they can infect another cell.
Immunity can be produced by receiving a vaccine. Vaccines are made to resemble a specific virus, such as the virus that causes influenza or measles, which can be given to people without causing the disease. In response to a vaccine, the body increases the number of T and B lymphocytes that are able to recognise the specific virus. In this way, vaccines can produce immunity to a specific virus.
Many vaccines are available to prevent common and severe infections, including influenza, measles, mumps, polio, chickenpox (varicella), rabies, German measles (rubella), Hepatitis A and B, Japanese encephalitis, and yellow fever. However, sometimes a virus changes (mutates) to evade the vaccine antibody, and re-vaccination becomes necessary.
Immediate protection against a viral infection can be achieved by receiving an injection or infusion of immunoglobulin. Immunoglobulin contains antibodies that were produced by another person or by an animal. For instance, a person traveling to an area where hepatitis A is prevalent may be given an injection of hepatitis A immuno-globulin. However, immunoglobulin can make some vaccines, such as measles or polio, less effective if given at the same time as the vaccine.
Drugs that combat a viral infection are called antiviral drugs. There are far fewer antiviral drugs than there are antibacterial drugs (antibiotics). Compared with most antibiotics, antiviral drugs are generally more difficult to design, more specific for their targeted organism, and generally more toxic. Antibiotics are not effective for viral infections, but if a person has a bacterial infection as well as a viral infection, an antibiotic is often necessary.
Respiratory Viral Infections
Probably the most common viral infections are those of the lungs and airways. These illnesses include the common cold, influenza, throat infection (pharyngitis or laryngitis), croup in small children, and inflammation of the windpipe (tracheitis) or other airways (bronchiolitis, bronchitis).
The common cold is a viral infection of the lining of the nose, sinuses, throat, and large airways.
Many different viruses cause colds. Picorna-viruses, such as the rhinoviruses, cause most spring, summer, and fall colds. Influenza viruses and respiratory syncytial viruses, which appear regularly in the late fall and winter, cause a spectrum of illnesses, including colds. Influenza viruses spread easily from person to person in infected droplets that are coughed or sneezed into the air. Rhinoviruses and respiratory syncytial viruses also are spread this way, but perhaps mainly by direct contact with infected secretions carried on the fingers.
Why a person is more likely to become infected at one time than another is not entirely known. Becoming chilled doesn't by itself cause colds or increase a person's susceptibility to infection by a respiratory virus. A person's general health or eating habits don't seem to make any difference either. Neither does having an abnormality of the nose or throat, such as enlarged tonsils or adenoids. However, people who are fatigued or emotionally distressed, those who have allergies of the nose or throat, and women who are halfway between menstrual periods may be more likely to notice the symptoms of a cold.
Symptoms and Complications
Symptoms of the common cold start one to three days after infection. Usually, the first symptoms are discomfort in the nose or throat. Later the person starts sneezing, has a runny nose, and feels mildly ill. Usually, fever does not develop, but sometimes a slight fever appears when symptoms are beginning. Secretions from the nose are watery and clear and can be annoyingly plentiful during the first day or two. Later the secretions become thicker, opaque, yellow-green, and less abundant. Many people also develop a cough. Symptoms usually disappear in four to 10 days, although a cough with or without sputum often lasts into the second week.
Complications may prolong the symptoms. Infection of the windpipe (trachea) along with some tightness in the chest and a burning discomfort are more likely in some people and with some viruses. People with persistent bronchitis or asthma may have more difficulty breathing during and after a cold. A bacterial infection of the ears, the sinuses, or the windpipe and airways (tracheobronchial infection) may follow the cold and requires treatment with antibiotics.
Most colds can be readily diagnosed based on the typical symptoms. However, bacterial infections, allergies, and other disorders can cause similar symptoms. The same viruses that cause colds also can cause symptoms similar to those of influenza. A high fever suggests that the infection is not a simple cold. Tests to diagnose a cold usually are not needed unless complications develop.
A person with a cold should stay warm and comfortable and try to avoid spreading the infection to others. This is often most readily done in the earliest stages of infection. Anyone with a fever or severe symptoms should probably rest at home. Drinking fluids helps to keep secretions loose and easier to expel.
Cold remedies are popular, but most of their benefits are doubtful. Aspirin may even increase virus shedding, while improving symptoms only slightly. If a drug is needed to relieve pain or fever in a child or adolescent, acetaminophen or ibuprofen is preferred, because aspirin is associated with an increased risk of Reye's syndrome, a potentially fatal condition.
A nasal decongestant provides only temporary, limited relief. Antihistamines may help dry up a runny nose, but they have been shown to do so only in people with a history of allergy and they cause drowsiness and other side effects, particularly in the elderly. Inhaling steam or mist from a vaporizer is a method some find useful to loosen secretions and reduce chest tightness, and washing the nasal passages with an isotonic salt solution can help to remove tenacious secretions.
Because coughing may be the only way to clear secretions and debris from the airways during a viral infection, it is preferable to leave a cough untreated unless it interferes with sleep or causes great discomfort. A severe cough may be treated with a cough suppressant. Antibiotics aren't effective against a cold; they should be used only if a bacterial infection also develops.
Influenza (flu) is a viral infection that causes a fever, runny nose, cough, headache, a feeling of illness (malaise), and inflammation of the lining of the nose and airways.
Every year, widespread outbreaks of respiratory illness caused by influenza occur during late fall or early winter. The illness occurs throughout the world. Although many respiratory viruses can cause the symptoms of influenza, the influenza A or influenza B virus is usually responsible for the epidemics in the late fall or winter.
The virus is spread by inhaling infected droplets that have been coughed or sneezed out by an infected person or by having direct contact with an infected person's secretions. Handling infected household articles may sometimes be responsible.
Influenza differs from the common cold. Symptoms start 24 to 48 hours after infection and can begin suddenly. Chills or a chilly sensation may be the initial indication of influenza. Fever is common during the first few days, and the temperature may rise to 102° to 103° F. Many people feel sufficiently ill to remain in bed; they have aches and pains throughout the body, most pronounced in the back and legs.
Headache is often severe, with aching around and behind the eyes. Bright light may make the headache worse.
At first, the respiratory symptoms may be relatively mild, with a scratchy sore throat, a burning sensation in the chest, a dry cough, and a runny nose. Later, the cough can become severe and bring up sputum. The skin may be warm and flushed, especially on the face. The mouth and throat may redden, the eyes may water, and the whites of the eyes may be mildly inflamed. The ill person, especially a child, may have nausea and vomiting.
After two or three days, most symptoms disappear rapidly, and the fever usually ends, although fever sometimes lasts up to five days. However, bronchitis and coughing may persist for 10 days or longer, and changes in the airways may take 6 to 8 weeks to completely resolve. Weakness and fatigue may persist for several days or occasionally for weeks.
Because most people are familiar with the symptoms of influenza, and because influenza occurs in epidemics, influenza is often correctly diagnosed by the person who has it or by family members. The severity of the illness and the presence of a high fever distinguish influenza from the common cold. Although not always necessary or available, a blood test can identify an influenza infection. Even better at establishing the diagnosis of influenza is the recovery of the virus in a culture of respiratory secretions.
A person exposed to an influenza virus produces antibodies, which protect against reinfection by that particular virus. Vaccination against influenza every year, however, is the best way to avoid contracting influenza. Influenza vaccines contain inactivated (or "killed") influenza virus strains or viral particles. A vaccine may be monovalent (one strain) or polyvalent (usually three strains). A monovalent vaccine permits a larger dose against a new virus strain, and a polyvalent vaccine boosts resistance against more than one strain. A different vaccine is introduced every year based on predictions of which strains are most likely to cause influenza. The predictions take into account the strain of virus that predominated during the previous flu season and the strain causing disease in other parts of the world in the current preseason.
Vaccination is particularly important for those who are likely to become very ill if infected. In the United States, vaccination should take place during the fall, so that levels of antibodies will be highest during the peak influenza months–November through March. For most people, about two weeks are needed for the vaccination to provide protection. However, children or other people who have never been exposed to an influenza virus need to receive two doses of vaccine, separated by one month, for adequate immunity.
Amantadine or rimantadine, two antiviral drugs, can protect against Influenza A but not Influenza B. They are used during epidemics of Influenza A to protect those in close contact with infected people and others at high risk who have not been vaccinated. The drug may be discontinued 2 to 3 weeks after a person is vaccinated. If a vaccine can not be given, amantadine or rimantadine is taken throughout the epidemic, usually for 6 to 8 weeks. These drugs can cause nervousness, sleeplessness, and other side effects, especially in the elderly and in those with brain or kidney disease. Rimantadine tends to cause fewer side effects than amantadine.
The main treatment for influenza is to stay in bed or rest adequately, to maintain hydration by drinking plenty of fluids, and to avoid exertion, ideally from the time the symptoms begin until 24 to 48 hours after the body temperature returns to normal. People with severe symptoms but without complications may take acetaminophen, aspirin, ibuprofen, or naproxen. Because of the danger of Reye's syndrome, children should not be given aspirin. Acetaminophen is, however, acceptable for use in children if needed. Other measures as listed for the common cold, such as nasal decongestants and steam inhalation, may relieve symptoms.
If taken early in the course of uncomplicated Influenza A infection, amantadine or rimantadine helps to reduce the duration and severity of fever and respiratory symptoms. Neither reduces the severity of viral pneumonia, but either may be given to try to improve the likelihood of recovery. Ribavirin, which can be inhaled as an aerosol or taken orally, has been demonstrated to shorten the duration of the fever and affect the virus ability to reproduce, but its use is still experimental. Ribavirin may, however, be given to relieve the symptoms of viral pneumonia.
A secondary bacterial infection is treated with antibiotics. Bacterial pneumonia caused by one type of bacteria, the pneumococcus, may be prevented with a polyvalent vaccine containing the common types of pneumococci. However, the vaccine is not given to someone who already has influenza.
- Viral Infections provides quality information about viral infections
- Infections - bacterial and viral: Better Health Channel
- Viral Infections: Queensland Government
- Diseases triggered by viral infections: Health Insite
Similar of Viral Infections