Viruses are the smallest of parasites, and are completely dependent on the cells they infect for their reproduction. Viruses are composed of an outer coat of protein, which is sometimes surrounded by a lipid envelope, and an inner nucleic acid core consisting of either RNA or DNA. Generally, after docking with the plasma membrane of a susceptible cell, the viral core penetrates the cell membrane to initiate the viral infection. After infecting cells, viruses commandeer the cell's molecular machinery to direct their own replication and packaging. The “replicative phase” of the viral lifecycle may begin immediately upon entry into the cell, or may occur after a period of dormancy or latency. After the infected cell synthesizes sufficient amounts of viral components, the “packaging phase” of the viral life cycle begins and new viral particles are assembled. Some viruses reproduce without killing their host cells, and many of these bud from host cell membranes. Other viruses cause their host cells to lyse or burst, releasing the newly assembled viral particles into the surrounding environment, where they can begin the next round of their infectious cycle.
Several hundred different types of viruses are known to infect humans, however, since many of these have only recently been recognized, their clinical significance is not fully understood. Of these viruses that infect humans, many infect their hosts without producing overt symptoms, while others (e.g., influenza) produce a well-characterized set of symptoms. Importantly, although symptoms can vary with the virulence of the infecting strain, identical viral strains can have drastically different effects depending upon the health and immune response of the host. Despite remarkable achievements in the development of vaccines for certain viral infections (i.e., polio and measles), and the eradication of specific viruses from the human population (e.g., smallpox), viral diseases remain as important medical and public health problems. Indeed, viruses are responsible for several “emerging” (or reemerging) diseases (e.g., West Nile encephalitis & Dengue fever), and also for the largest pandemic in the history of mankind (HIV and AIDS).
Viruses that primarily infect humans are spread mainly via respiratory and enteric excretions. These viruses are found worldwide, but their spread is limited by inborn resistance, prior immunizing infections or vaccines, sanitary and other public health control measures, and prophylactic antiviral drugs. Zoonotic viruses pursue their biologic cycles chiefly in animals, and humans are secondary or accidental hosts. These viruses are limited to areas and environments able to support their nonhuman natural cycles of infection (vertebrates or arthropods or both). However, with increased global travel by humans, and the likely accidental co-transport of arthropod vectors bearing viral payloads, many zoonotic viruses are appearing in new areas and environments as emerging diseases. For example, West Nile virus, which is spread by the bite of an infected mosquito, and can infect people, horses, many types of birds, and other animals, was first isolated from a febrile adult woman in the West Nile District of Uganda in 1937. The virus made its first appearance in the Western Hemisphere, in the New York City area in the autumn of 1999, and during its first year in North America, caused the deaths of 7 people and the hospitalization of 62. At the time of this writing (August, 2002) the virus has been detected in birds in 37 states and the District of Columbia, and confirmed human infections have occurred in Alabama, the District of Columbia, Florida, Illinois, Indiana, Louisiana, Massachusetts, Mississippi, Missouri, New York City, Ohio, and Texas. (See: http://www.cdc.gov/od/oc/media/wncount.htm).
Additionally, some viruses are known to have oncogenic properties. Human T-cell lymphotropic virus type 1 (a retrovirus) is associated with human leukemia and lymphoma. Epstein-Barr virus has been associated with malignancies such as nasopharyngeal carcinoma, Burkitt's lymphoma, Hodgkin's disease, and lymphomas in immunosuppressed organ transplant recipients. Kaposi's sarcoma-associated virus is associated with Kaposi's sarcoma, primary effusion lymphomas, and Castleman's disease (a lymphoproliferative disorder).
Treatment of viral diseases presents unique challenges to modern medicine. Since viruses depend on host cells to provide many functions necessary for their multiplication, it is difficult to inhibit viral replication without at the same time affecting the host cell itself. Consequently, antiviral treatments are often directed at the functions of specific enzymes of particular viruses. However, such antiviral treatments that specifically target viral enzymes (e.g., HIV protease, or HIV reverse transcriptase) often have limited usefulness, because resistant strains of viruses readily arise through genetic drift and mutation.