Influenza, also known as the flu, is a highly contagious viral infection that affects the respiratory system. The flu is caused by the influenza virus, which is a member of the Orthomyxoviridae family of viruses. The flu virus is classified into three types: A, B, and C. Type A viruses are further divided into subtypes based on their surface proteins (hemagglutinin [HA] and neuraminidase [NA]). Type B viruses are not subtyped. There are many different strains of the flu virus circulating at any given time, and these strains can change over time.
The flu is spread through contact with respiratory secretions from an infected person, typically through coughing or sneezing. The virus can also be spread indirectly by touching surfaces that have been contaminated with the virus and then touching one’s nose or mouth. People with the flu are most contagious during the first 3-5 days of their illness; however, they can remain infectious for up to 7 days after becoming sick.
Symptoms of the flu include fever (usually 101°F or higher), headache, muscle aches and pains, fatigue, sore throat, runny or stuffy nose, and cough. Some people may also experience vomiting and diarrhea. Symptoms usually appear 2-3 days after exposure to the virus and can last for up to 2 weeks. Most people recover from the flu without complications; however, some people may develop severe illness such as pneumonia or bronchitis. This is more likely to occur in young children, older adults (>65 years), pregnant women, American Indians/Alaska Natives ,and people with certain chronic medical conditions (such as heart disease , diabetes , asthma , COPD ,or cancer ). Influenza vaccination is recommended for everyone 6 months of age or older . Vaccination is especially important for people at high risk for developing serious complications from influenza . These include pregnant women ; young children ; people 65 years of age or older ; American Indians/Alaska Natives ; people who live in nursing homes or other long-term care facilities ;people with certain chronic medical conditions (such as heart disease , diabetes , asthma) ;and health care workers .