Influenza is most frequently presented as a respiratory illness. Symptoms of flu include fever, headache, extreme fatigue, dry cough, sore throat, runny or stuffy nose, and muscle aches.  Children can have additional gastro-intestinal symptoms, such as nausea, vomiting, and diarrhoea, but these symptoms are uncommon in adults.Although the term ※stomach flu§ is sometimes used to describe vomiting, nausea, or diarrhoea, these illnesses are usually caused by certain other viruses, bacteria, or possibly parasites,
but rarely by the influenza virus.

The most frequent complication of influenza infection is pneumonia, most commonly secondary bacterial pneumonia (e.g. Streptococcus pneumonia, Haemophilus influenza, or Staphylococcus aureus). Primary influenza viral pneumonia is an uncommon complication with a high fatality rate. Reye syndrome is a complication that occurs almost exclusively in children taking aspirin. This condition is primarily associated with influenza B (or varicella zoster), and presents with severe vomiting and confusion which may progress to coma, due to swelling of the brain. 

Other complications include myocarditis (inflammation of the heart), and worsening of chronic bronchitis and other chronic pulmonary diseases.  Death is reported in 0.5 每1 per 1000 cases. The majority of deaths occur in persons ≡ 65 years of age.

Influenza is a single-stranded, helically shaped, RNA virus of the orthomyxovirus family. Basic antigen types A, B, and C are determined by the nuclear material. Type A influenza has subtypes that are determined by the surface antigens hemagglutinin (H) and neuraminidase (N).  Three types of hemagglutinin in humans (H1, H2, and H3) have a role in virus attachment to cells. Two types of neuraminidase (N1 and N2) have a role in virus penetration into cells.

Influenza A causes moderate to severe illness (including death), and affects all age groups.  The virus infects humans and other animals, such as pigs and birds.

Influenza B generally causes milder disease than type A, and primarily affects children. Influenza B is more stable than influenza A, with less antigenic drift and consequent immunologic stability.  It affects only humans.

Influenza C is rarely reported as a cause of human illness probably because most cases are subclinical. It has not been associated with epidemic disease.

New influenza strains spread rapidly in children in school and cr豕ches, nursing homes and in places where people crowd together. Influenza epidemics typically cause economically significant absenteeism. 

Influenza viruses can change in two different ways. One is called ※antigenic drift §. These are small changes in the virus that happen continually over time. Antigenic drift produces new viral strains that may not be recognized by the body＊s immune system. This process works as follows: a person infected with a particular flu virus strain develops antibody against that virus.  As newer viral strains appear, the antibodies against the older strains no longer recognize the ※newer§ virus, and re-infection can occur. This is one of the main reasons why people can get the flu more than one time. 

The other type of change is called ※antigenic shift§. Antigenic shift is an abrupt, major change in the influenza A viruses, resulting in new hemagglutinin and/or new hemagglutinin and neuraminidase viral proteins. Shift results in a new influenza A subtype. When shift happens, most people have little or no protection against the new virus. While influenza viruses are changing by antigenic drift all the time, antigenic shift happens only occasionally and can result in an influenza pandemic. Type A viruses undergo both kinds of changes; influenza type B viruses change only by the more gradual process of antigenic drift.