SynonymsFlu, the flu
EM of influenza virus.jpg
Influenza virus, magnified approximately 100,000 times
SpecialtyInfectious disease
SymptomsFever, runny nose, sore throat, muscle pains, headache, coughing, sneezing, feeling tired[1]
Usual onsetTwo days after exposure[1]
Duration~1 week[1]
CausesInfluenza viruses[2]
PreventionHandwashing, surgical mask, influenza vaccine[1][3]
MedicationAntiviral drugs such as oseltamivir[1]
Frequency3–5 million per year[1]
Deaths~375,000 per year[1]

Influenza, commonly known as the flu, is an infectious disease caused by an influenza virus.[1] Symptoms can be mild to severe.[4] The most common symptoms include: high fever, runny nose, sore throat, muscle pains, headache, coughing, sneezing, and feeling tired.[1] These symptoms typically begin two days after exposure to the virus and most last less than a week.[1] The cough, however, may last for more than two weeks.[1] In children, there may be diarrhea and vomiting, but these are not common in adults.[5] Diarrhea and vomiting occur more commonly in gastroenteritis, which is an unrelated disease and sometimes inaccurately referred to as "stomach flu" or the "24-hour flu".[5] Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.[2][4]

Three of the four types of influenza viruses affect people, Type A, Type B, and Type C.[2][6] Type D has not been known to infect people, but is believed to have the potential to do so.[6][7] Usually, the virus is spread through the air from coughs or sneezes.[1] This is believed to occur mostly over relatively short distances.[8] It can also be spread by touching surfaces contaminated by the virus and then touching the mouth or eyes.[4][8] A person may be infectious to others both before and during the time they are showing symptoms.[4] The infection may be confirmed by testing the throat, sputum, or nose for the virus.[2] A number of rapid tests are available; however, people may still have the infection even if the results are negative.[2] A type of polymerase chain reaction that detects the virus's RNA is more accurate.[2]

Frequent hand washing reduces the risk of viral spread.[3] Wearing a surgical mask is also useful.[3] Yearly vaccinations against influenza are recommended by the World Health Organization for those at high risk.[1] The vaccine is usually effective against three or four types of influenza.[1] It is usually well-tolerated.[1] A vaccine made for one year may not be useful in the following year, since the virus evolves rapidly.[1] Antiviral drugs such as the neuraminidase inhibitor oseltamivir, among others, have been used to treat influenza.[1] The benefit of antiviral drugs in those who are otherwise healthy do not appear to be greater than their risks.[9] No benefit has been found in those with other health problems.[9][10]

Influenza spreads around the world in yearly outbreaks, resulting in about three to five million cases of severe illness and about 250,000 to 500,000 deaths.[1] About 20% of unvaccinated children and 10% of unvaccinated adults are infected each year.[11] In the northern and southern parts of the world, outbreaks occur mainly in the winter, while around the Equator, outbreaks may occur at any time of the year.[1] Death occurs mostly in the young, the old, and those with other health problems.[1] Larger outbreaks known as pandemics are less frequent.[2] In the 20th century, three influenza pandemics occurred: Spanish influenza in 1918 (~50 million deaths), Asian influenza in 1957 (two million deaths), and Hong Kong influenza in 1968 (one million deaths).[12] The World Health Organization declared an outbreak of a new type of influenza A/H1N1 to be a pandemic in June 2009.[13] Influenza may also affect other animals, including pigs, horses, and birds.[14]

Signs and symptoms

Most sensitive symptoms for diagnosing influenza[15]
Symptom: sensitivity specificity
Fever 68–86% 25–73%
Cough 84–98% 7–29%
Nasal congestion 68–91% 19–41%

  • All three findings, especially fever, were less sensitive in people over 60 years of age.

Symptoms of influenza,[16] with fever and cough the most common symptoms.[15]

Approximately 33% of people with influenza are asymptomatic.[17]

Symptoms of influenza can start quite suddenly one to two days after infection. Usually the first symptoms are chills and body aches, but fever is also common early in the infection, with body temperatures ranging from 38 to 39 °C (approximately 100 to 103 °F).[18] Many people are so ill that they are confined to bed for several days, with aches and pains throughout their bodies, which are worse in their backs and legs.[19]

Symptoms of influenza

It can be difficult to distinguish between the common cold and influenza in the early stages of these infections.[24] Influenza is a mixture of symptoms of common cold and pneumonia, body ache, headache, and fatigue. Diarrhea is not usually a symptom of influenza in adults,[15] although it has been seen in some human cases of the H5N1 "bird flu"[25] and can be a symptom in children.[21] The symptoms most reliably seen in influenza are shown in the adjacent table.[15]

Since antiviral drugs are effective in treating influenza if given early (see treatment section, below), it can be important to identify cases early. Of the symptoms listed above, the combinations of fever with cough, sore throat and/or nasal congestion can improve diagnostic accuracy.[26] Two decision analysis studies[27][28] suggest that during local outbreaks of influenza, the prevalence will be over 70%,[28] and thus patients with any of these combinations of symptoms may be treated with neuraminidase inhibitors without testing. Even in the absence of a local outbreak, treatment may be justified in the elderly during the influenza season as long as the prevalence is over 15%.[28]

The available laboratory tests for influenza continue to improve. The United States Centers for Disease Control and Prevention (CDC) maintains an up-to-date summary of available laboratory tests.[29] According to the CDC, rapid diagnostic tests have a sensitivity of 50–75% and specificity of 90–95% when compared with viral culture.[30]

Occasionally, influenza can cause severe illness including primary viral pneumonia or secondary bacterial pneumonia.[31][32] The obvious symptom is trouble breathing. In addition, if a child (or presumably an adult) seems to be getting better and then relapses with a high fever, that is a danger sign since this relapse can be bacterial pneumonia.[33]

Sometimes, influenza may have abnormal presentations, like confusion in the elderly and a sepsis-like syndrome in the young.[34] Encephalitis due to the flu is rare but not unheard of.

Emergency warning signs

Signs of dehydration

  • (in infants) Far fewer wet diapers than regularly[35]
  • Cannot keep down fluids
  • (in infants) No tears when crying.
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dansk: Influenza
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Diné bizaad: Tahoniigááh
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íslenska: Inflúensa
italiano: Influenza
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Bahasa Melayu: Influenza
မြန်မာဘာသာ: တုပ်ကွေးရောဂါ
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norsk: Influensa
norsk nynorsk: Influensa
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Simple English: Influenza
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