Influenza, also known as flu, contagious infection primarily of the respiratory tract. Influenza is sometimes referred to as grippe. Influenza is caused by a virus transmitted from one person to another in droplets coughed or sneezed into the air. It is characterized by coldlike symptoms plus chills, fever, headaches, muscle aches, and fatigue. Most people recover completely in about a week. But some people are vulnerable to complications such as bronchitis and pneumonia.
In addition to humans, influenza occurs in pigs, horses, and several other mammals as well as in certain wild and domesticated birds. At least some influenza viruses can jump from one species to another.
Today scientists know that members of the family Orthomyxoviridae, a group of viruses that infect vertebrate animals, cause influenza. The virus consists of an inner core of the genetic material ribonucleic acid (RNA) surrounded by a protein coat and an outer lipid (fatty) envelope. From this envelope, spikes of proteins called hemagglutinin and neuraminidase stick out. Hemagglutinin enables the virus to bind to and invade cells, and neuraminidase allows the virus to move among cells. But these proteins also act as antigens—that is, they are recognized as foreign matter by the human or other host organism, and this recognition triggers an immune response in the host.
The word influenza is derived from the Latin word influentia. Italians in the early 16th century first applied the word influenza to outbreaks of any epidemic disease because they blamed such outbreaks on the influence of heavenly bodies. The first known use of the name specifically for the flu occurred in 1743 when an epidemic swept through Rome and its environs.
There are three types of influenza viruses, known as A, B, and C. Type A, the most dangerous, infects a wide variety of mammals and birds. It causes the most cases of the disease in humans and is the type most likely to become epidemic. Type B infects humans and birds, producing a milder disease that can also cause epidemics. Type C apparently infects only humans. It typically produces either a very mild illness indistinguishable from a common cold or no symptoms at all. Type C does not cause epidemics.
Once a person has been infected by a specific strain of influenza, he or she has built up immunity to that strain in the form of antibodies. The person’s immune system then can recognize the strain’s hemagglutinin or neuraminidase and attack them if they reappear. The antibodies offer some protection against antigenic drifts, but not against antigenic shifts. Thus, because the viruses continually change, they can cause repeated waves of infection, even among people previously infected.
Influenza viruses pass from person to person mainly in droplets expelled during sneezes and coughs. When a person breathes in virus-laden droplets, the hemagglutinin on the surface of the virus binds to enzymes in the mucous membranes that line the respiratory tract. The enzymes, known as proteases, cut the hemagglutinin in two, which enables the virus to gain entry into cells and begin to multiply. These proteases are common in the respiratory and digestive tracts but not elsewhere, which is why the flu causes primarily a respiratory illness with occasional gastrointestinal symptoms.
Symptoms and Diagnosis:
Influenza is an acute disease with a rapid onset and pronounced symptoms. After the influenza virus invades a person’s body, an incubation period of one to two days passes before symptoms appear. Classic symptoms include sore throat, dry cough, stuffed or runny nose, chills, fever with temperatures as high as 39º C (103º F), aching muscles and joints, headache, loss of appetite, occasional nausea and vomiting, and fatigue. For most people flu symptoms begin to subside after two to three days and disappear in seven to ten days. However, coughing and fatigue may persist for two or more weeks.
Treatment and Prevention:
There is no specific cure for influenza. Recommended treatment usually consists of bed rest and increased intake of nonalcoholic fluids until fever and other symptoms lessen in severity. Certain drugs have been found effective in lessening flu symptoms, but medical efforts against the disease focus chiefly on prevention by means of vaccines that create immunity.