Viral Infections
Viruses are among the smallest microbes, much smaller even than bacteria. Viruses are not cells. They consist of one or more molecules of DNA or RNA, which contain the virus's genes surrounded by a protein coat.
Unlike most bacteria, most viruses do cause disease because they invade living, normal cells, such as those in the human body. They then multiply and produce other viruses like themselves. Each virus is very particular about which cell it attacks. Various human viruses specifically attack particular cells in the body's organs, systems, or tissues, such as the liver, respiratory system, or blood cells.
Many viral infections do not result in disease. For example, by the time most people in the United States become adults, they have been infected by cytomegalovirus (CMV). Most of these people, however, do not develop CMV disease symptoms. Other viral infections can result in deadly diseases, such as HIV infection, which causes acquired immunodeficiency syndrome (AIDS).
Viruses usually infect one particular type of cell. For example, cold viruses infect only cells of the upper respiratory tract. Additionally, most viruses infect only a few species of plants or animals; some infect only people.
Viruses are transmitted in a variety of ways. Some are swallowed, some are inhaled, and some are transmitted by the bites of insects and other parasites (for example, mosquitoes and ticks).
The body has a number of defenses against viruses. Physical barriers, such as the skin, discourage easy entry. Infected cells also make interferons, substances that can make noninfected cells more resistant to infection by many viruses.
Upon entering the body, a virus triggers the body's immune defenses. These defenses begin with white blood cells, such as lymphocytes, which learn to attack and destroy the virus or the cells it has infected. If the body survives the virus attack, the lymphocytes "remember" the invader and are able to respond more quickly and effectively to a subsequent infection by the same virus. This is called immunity. Immunity can also be produced by receiving a vaccine.
Drugs that combat viral infections are called antiviral drugs. Antiviral drugs work by interfering with viral replication. Because viruses are tiny and replicate inside cells using the cells' own metabolic pathways, there are only a limited number of metabolic functions that antiviral drugs can target. In contrast, bacteria are relatively large organisms, commonly reproduce by themselves outside of cells, and have many metabolic functions against which antibiotics can be directed. Therefore, antiviral drugs are much more difficult to develop. Antiviral drugs can be toxic to human cells. Viruses can develop resistance to antiviral drugs.
Antibiotics are not effective against viral infections, but if a person has a bacterial infection in addition to a viral infection, an antibiotic is often necessary.
Probably the most common viral infections are those of the nose, throat, and airways. These infections include sore throat, sinusitis, the common cold, and influenza. Doctors often refer to these as upper respiratory infections (URIs). In small children, viruses also commonly cause croup and inflammation of the windpipe (laryngitis) or other airways deeper inside the lungs (bronchiolitis, bronchitis).
Some viruses (for example, rabies, West Nile virus, and several different encephalitis viruses) infect the nervous system. Viral infections also develop in the skin, sometimes resulting in warts or other blemishes. Additionally, many viruses commonly infect infants and children.
Other common viral infections are caused by the herpesviruses. Eight different herpesviruses infect people. Three of these—herpes simplex virus type 1, herpes simplex virus type 2, and varicella-zoster virus—cause infections that produce blisters on the skin. Another herpesvirus, Epstein-Barr virus, causes infectious mononucleosis. Cytomegalovirus is a cause of serious infections in newborns and in people with a weakened immune system. It can also produce an illness similar to infectious mononucleosis in people with a healthy immune system. Human herpesviruses 6 and 7 cause a childhood illness known as roseola infantum (see Viral Infections: Roseola Infantum). Human herpesvirus 8 has been implicated as a cause of cancer (Kaposi's sarcoma) in people with AIDS.
All of the herpesviruses cause lifelong infection because the virus remains within its host cell in a dormant (latent) state. Sometimes, the virus reactivates and produces further episodes of disease. Reactivation may occur rapidly or many years after the initial infection.
Information obtained from National Institute of Health