An acute respiratory viral infection characterized by fever, chills, sore throat, headache, body aches, and severe cough. The term flu, which is frequently used incorrectly for various respiratory and even intestinal illnesses (such as stomach flu), should be used only for illness with these classic symptoms. The onset is typically abrupt, in contrast to common colds which begin slowly and progress over a period of days. Influenza is usually epidemic in occurrence. The first documented pandemic, or global epidemic, of influenza is considered to have been in 1580. The influenza pandemic of 1918, the most famous occurrence, was responsible for at least 20 million deaths worldwide.

The three types of influenza viruses, types A, B, and C, are classified in the virus family Orthomyxoviridae, and they are similar, but not identical, in structure and morphology. Types A and B are more similar in physical and biologic characteristics to each other than they are to type C. Influenza viruses may be spherical or filamentous in shape, and they are of medium size among common viruses of humans. Animal virus

When a cell is infected by two similar but different viruses of one type, especially type A, various combinations of the original parental viruses may be packaged or assembled into the new progeny; thus, a progeny virus may be a mixture of gene segments from each parental virus and therefore may gain a new characteristic, for example, a new surface protein. This phenomenon is called genetic reassortment, and the frequency with which it occurs and leads to viruses with new features is a significant cause of the constant appearance of new variants of the virus. In the laboratory, reassortment occurs between animal and human strains as well as between human strains. It probably occurs in nature also, and is thought to contribute to the appearance of new strains that infect humans. Generally, if a new variant is sufficiently different from the vaccine currently in use, the vaccine will provide limited or no protection.

The influenza virus has a short incubation period; that is, there is only a period of 1–3 days between infection and symptoms, and this leads to the abrupt development of symptoms that is a hallmark of influenza infections. The virus is typically shed in the throat for 5–7 days. Complete recovery from uncomplicated influenza usually takes several days, and the individual may feel weak and exhausted for a week or more after the major symptoms disappear. The two main complications of influenza are primary influenza virus pneumonia and secondary bacterial pneumonia. Primary influenza pneumonia is relatively infrequent, occurring in less than 1% of cases during an epidemic, although mortality may be 25–30%. The damage to epithelial cells and subsequent loss of the ability to clear particles from the respiratory tract can lead to secondary bacterial pneumonia. This problem commonly occurs in elderly individuals or those with underlying chronic lung disease or similar problems. Influenza-induced pneumonia may cause as many as 20,000 deaths in a typical influenza season. Another complication, known as Reye's syndrome, may follow influenza, and is more common in children. This disease of the brain develops within 2–12 days of a systemic viral infection, and can result in vomiting, liver damage, coma, and sometimes death.

All three types of influenza viruses can cause disease in humans, but there are significant differences in severity of the disease and the range of hosts. In contrast to the large number of animal species infected by type A virus, types B and C are only rarely isolated from animals and infect predominantly humans.

The presence or absence of antibodies is very important in the epidemiology of influenza. In individuals with no immunity, attack rates may reach 70% and severe illness may result. Even low levels of antibody may provide partial protection in an individual and decrease the severity of the illness to only coldlike symptoms.

During an epidemic, one strain of influenza is predominant, but it is not unusual for two or more other strains to be present as minor infections in a population. Outbreaks of influenza occur during cold-weather months in temperate climates, and typically most cases cluster on a period of 1–2 months, in contrast to broader periods of illness with many other respiratory viruses. An increased death rate due to primary pneumonia and bacterial superinfection is common and is one of the ways that public health authorities monitor an epidemic.

Control and prevention of influenza are attempted through the use of drugs and vaccines. Inactivated viral vaccines are used to prevent influenza, although use of attenuated live strains of the virus may better stimulate the cell-mediated immune response and provide higher-quality and longer-lasting immunity. The makeup of the vaccine is modified annually, based upon predictions of the expected prevalent strain for each flu season, but usually contains antigens of two type A viruses and one type B virus. These vaccines take advantage of the natural ability of the viral nucleic acid to reassort and form new strains. The vaccines utilize strains that are not virulent and will replicate at lower temperatures, as found in the nasopharnyx, but not at higher temperatures as found in the lower respiratory tract. The techniques of modern biotechnology are employed to clone copies of parts of the virus or to provide oligonucleotides corresponding to crucial functional areas of the virus, to obtain improved protection and reduced side effects. Biotechnology