Influenza, commonly called "the flu," is an illness caused by RNA viruses that infect the respiratory tract of many animals, birds, and humans. In most people, the infection results in the person getting a fever, cough, headache, and malaise (tired, no energy); some people also may develop a sore throat, nausea, vomiting, and diarrhea. The majority of individuals has flu symptoms for about one to two weeks and then recovers with no problems. However, compared with most other viral respiratory infections, such as the common cold, influenza (flu) infection can cause a more severe illness with a mortality rate (death rate) of about 0.1% of people who are infected with the virus.

    The above is the usual situation for the yearly occurring "conventional" or "seasonal" flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when a portion of the human population is exposed to a flu strain against which the population has little or no immunity because the virus has become altered in a significant way. These outbreaks are usually termed epidemics. Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified in 1933. By an examination of preserved tissue, the worst influenza pandemic (also termed the Spanish flu or Spanish influenza) occurred in 1918 when the virus caused between 40-100 million deaths worldwide, with a mortality rate estimated to range from 2%-20%.

    In April 2009, a new influenza strain against which the world population has little or no immunity was isolated from humans in Mexico. It quickly spread throughout the world so fast that the WHO declared this new flu strain (first termed novel H1N1 influenza A swine flu, often later shortened to H1N1 or swine flu) as the cause of a pandemic on June 11, 2009. This was the first declared flu pandemic in 41 years. Fortunately, there was a worldwide response that included vaccine production, good hygiene practices (especially hand washing) were emphasized, and the virus (H1N1) caused far less morbidity and mortality than was expected and predicted. The WHO declared the pandemic's end on Aug. 10, 2010, because it no longer fit into the WHO's criteria for a pandemic.

    A new influenza strain, H3N2, was identified in 2011, but this strain has caused only about 330 infections with one death in the U.S. Another strain, H5N1, a bird flu virus, has been identified since 2003 and has caused about 650 human infections; this virus has not been detected in the U.S. and currently is not known to be easily spread among people in contrast to other flu strains. Unfortunately, people infected with H5N1 have a high death rate (about 60% of infected people die); currently, H5N1 is not readily transferred from person to person like other flu viruses.

    The most recent data for the mortality (death rates) from influenza rate (death rate) for the United States in 2016 indicates that mortality from influenza varies from year to year. Death rates estimated by the CDC range from about 12,000 during 2011-2012 to 56,000 during 2012-2013.

    Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until the virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children, and it occasionally causes ear, eye, sinus, joint, and a few other infections, but it does not cause the flu.

    Another confusing term is stomach flu. This term refers to a gastrointestinal tract infection, not a respiratory infection like influenza (flu); stomach flu (gastroenteritis) is not caused by influenza viruses.

    Although initially symptoms of influenza may mimic those of a cold, influenza is more debilitating with symptoms of fatigue, fever, and respiratory congestion. Colds can be caused by over 100 different virus types, but only influenza viruses (and subtypes) A, B, and C cause the flu. In addition, colds do not lead to life-threatening illnesses like pneumonia, but severe infections with influenza viruses can lead to pneumonia or even death.