Influenza

Influenza virus types A and B are both common causes of acute respiratory illnesses, although influenza A viruses are the principal cause of large epidemics, as well as pandemics. Recorded since the middle of the 18th century, new influenza A subtypes have caused global pandemics at unpredictable intervals.

Severe morbidity and mortality are more common among elderly people and in specific high-risk groups. Influenza viruses undergo frequent changes in their surface antigens.

Immunity resulting from infection by one influenza virus does not protect fully against antigenic or genetic variants of the same subtype (influenza A viruses) or type (influenza B viruses). As a consequence, influenza outbreaks occur every year. New influenza vaccines must be designed annually to match the circulating viruses which are expected to cause the next epidemic.

Influenza viruses spread from person to person, primarily through respiratory droplets.

General symptoms are generally localized to the respiratory tract and include nonproductive cough, fever, headache, malaise, muscle aches, rhinitis and sore throat. Illness without fever can occur, especially in older adults and infants. Complications of influenza virus infection include primary influenza viral pneumonia and secondary bacterial pneumonia, as well as ear and sinus infections and worsening of chronic medical conditions (congestive heart failure, asthma, diabetes).