Influenza, commonly called "the flu," is caused by viruses that infect the respiratory tract.

    Flu symptoms include：fever, chills, cough, sore throat, headache, muscle aches, and fatigue.

    The incubation period for flu is about one to four days.

    Flu is contagious, and symptoms may last up to seven to 14 days.

    Flu is diagnosed by the patient's history, physical exam, and laboratory tests.

    Flu is spread directly and indirectly; directly from person to person by airborne droplets produced during sneezing or coughing, for example, and indirectly when contaminated droplets land on surfaces that are subsequently touched by uninfected individuals.

    Influenza viruses are divided into three types, designated A, B, and C, with influenza A types usually causing the most problems in humans.

    Most people who get the conventional or seasonal flu recover completely in one to two weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia.

    The flu can make chronic health problems worse

    Much of the illness and death caused by conventional or seasonal influenza can be prevented by annual influenza vaccination.

    Influenza A undergoes frequent antigenic changes that require new vaccines to be developed and people to obtain a new vaccination every year. New vaccine technology is being developed.

    In April 2009, a new flu virus termed novel H1N1 swine flu developed in Mexico, rapidly spread worldwide, and caused the WHO to declare a flu pandemic. Eventually, the WHO declared the pandemic over in 2010. In 2012, a new type of flu strain developed, H3N2v, but has not developed into any epidemic situations currently.

    Effectiveness of the flu vaccine varies year to year because strains causing flu also vary yearly.

    Some home remedies may reduce flu symptoms.

    Like the influenza virus, drug treatments are constantly changing and improving, but currently, timely vaccination is still considered to be the best defense against the flu. However, the U.S. Centers for Disease Control and Prevention (CDC) considers antiviral drugs an important adjunct to the flu vaccine in the control of the disease process.

    People should be aware that flu pandemics can cause severe flu symptoms and sometimes cause death in many individuals who may be more susceptible to the pandemic flu than the conventional flu; however, the previous pandemic flu virus (H1N1) has been available in vaccines and is considered part of the conventional circulating flu viruses.

    Bird flu (H5N1) mainly infects birds, but it also infects humans who have close contacts with birds.

    Individuals should check with their doctors to determine if they are considered to be at higher risk of getting severe flu symptoms than the normally healthy population.