Influenza is a seasonal respiratory illness with a sudden onset. It is most often causes fever, body aches, and a dry cough. People with influenza often refer to it as "flu."

Flu season can span from October to May, and usually peaks between December and February.

There is some variety in presentations of flu. This depends on the age of a person and their general state of health. Many symptoms are commonly observed.

The Centers for Disease Control and Prevention (CDC) estimate that between 9.2 million and 35.6 million cases of flu have been reported to medical professionals in the United States (U.S.) every year since 2010.

Fast facts on flu symptoms

  • Flu season stretches from October to May.
  • Every year since 2010, between around 9 and 35 million people have visited the doctor regarding flu symptoms.
  • Flu symptoms include fever, cough, headache, and chills.
  • Most people recover from flu in 3 to 5 days.

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Fever is a common symptom of flu.

Typical signs and symptoms of the flu include:

Fever: A fever causes an increase in body temperature. A fever can be low-grade, in the 100° Fahrenheit (37.7° Celcius) range, or reach temperatures as high as 104°F (40°C). Young children tend to have higher temperatures and can even reach 106°F (41.1°C). Some people may describe feeling feverish and chilled at the same time. The fever may last up to a week.

Cough: At the outset of the infection, a cough brought on by flu is dry, persistent, and often painful. It may start mild but worsen as the infection evolves. A feeling of breathlessness or discomfort with breathing is common. A cough can persist for two weeks or more.

Headache: A flu headache is usually severe and tends to spread over the forehead and behind the eyes. It may be accompanied by additional eye symptoms, such as sensitivity to light, redness, tearing, and a burning sensation.

Chills: Fever can lead to shaking and a feeling of coldness.

Muscle aches and joint pain: Aches and pains can range from mild to severe. These sensations are common in the back, arms, and legs.

Malaise: Malaise is a general feeling of discomfort, illness, and uneasiness. The term refers to an overall lack of wellbeing. Appetite is also diminished or lacking altogether. This is a common complaint with many illnesses and is often the first indication of an infection.

Sore throat: A sore throat brought on by flu may be severe and typically lasts 3 to 5 days.

Weakness and fatigue: Weakness and fatigue can be so severe that they interfere with normal activities, such as attending work and school. People who have the flu cannot seem to get enough sleep and are often completely exhausted. Simple activities, such as getting up to use the bathroom, can seem challenging.

A runny or stuffy nose: Nasal congestion and sneezing are seen more often as symptoms of the common cold. However, the flu can cause these symptoms in some individuals.

Vomiting and diarrhea: These symptoms are rarely seen in adults with the flu, but vomiting or diarrhea occur in 10 to 20 percent of flu infections in children.

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The flu vaccine can help to keep influenza at bay. However, it is not 100 percent effective.

The majority of people recover from the flu in 3 to 5 days. The feeling of malaise may last for a week or longer.

However, flu can become complicated by pneumonia. Pneumonia is a serious lung infection that requires treatment and, in some cases, hospitalization. Older adult patients, young children, and those with underlying medical conditions are more likely to develop pneumonia.

It can be challenging to know if symptoms are flu-related or the result of another illness. The flu can resemble other viral respiratory infections, and symptoms can be less clear in older and younger populations.

The flu shot is not 100 percent effective, so if the above symptoms occur, visit a doctor. It can be easy to assume that symptoms are not flu-related after receiving a flu shot.

In 2015, a report from the Centers for Disease Control and Prevention (CDC) stated that the flu vaccine for that flu season was only 23 percent effective.

This low level of efficacy was attributed to a large number of "drift variants." Drift variants are strains of influenza that have changed to their genetic structure or infectious properties. These can differ from the viruses used to make the vaccine.

The vaccine would be ineffective against the newly formed pathogens.

An individual is likely to have the flu if they experience the following:

  • Flu occurs within 2 days of exposure to the virus, and there is a drastic and notable advancement of symptoms.
  • Fever lasts more than 1 to 2 days.
  • Every part of the body seems to hurt. Significant body aches all over indicate flu.
  • Flu-like symptoms are showing during flu season.
  • In adults, there is no vomiting or diarrhea. Children may have gastrointestinal symptoms.

If a person suspects flu, it is important to speak to a medical provider. Most people recover from the flu within a week. However, there are certain groups of people for whom the flu can be life-threatening.

There is a rapid test that can be carried out in the doctor's office to help confirm whether symptoms are flu-related. Antiviral medications are also available that can help reduce the level of infection and the severity of symptoms, so long as treatment is commenced during the first 48 hours of contact with the virus.