Influenza (also known as the flu) is a contagious respiratory illness caused by flu viruses. It can cause mild to severe illness. The flu is different from a cold. The flu usually comes on suddenly. People who have the flu often feel some or all of these symptoms:
Fever or feeling feverish/chills
Dry, persistent Cough
Runny or stuffy nose
Muscle or body aches
Fatigue (tiredness) and weakness
Aching muscles, especially in your back, arms and legs
Chills and sweats
Some people may have vomiting and diarrhea, though this is more common in children than adults.
It’s important to note that not everyone with flu will have a fever.
A wide range of complications can be caused by influenza virus infection of the upper respiratory tract (nasal passages, throat) and lower respiratory tract (lungs). While anyone can get sick with flu and become severely ill, some people are more likely to experience severe flu illness.
Sinus and ear infections are examples of moderate complications from flu, while pneumonia is a serious flu complication that can result from either influenza virus infection alone or from co-infection of flu virus and bacteria. Other possible serious complications triggered by flu can include inflammation of the heart (myocarditis), brain (encephalitis) or muscle (myositis, rhabdomyolysis) tissues, and multi-organ failure (for example, respiratory and kidney failure). Flu virus infection of the respiratory tract can trigger an extreme inflammatory response in the body and can lead to sepsis, the body’s life-threatening response to infection. Flu also can make chronic medical problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic heart disease may experience a worsening of this condition triggered by flu.