Influenza (commonly known as flu) is a highly contagious illness caused by a group of viruses called the orthomyxoviruses. Infection with these viruses leads to a self-limiting illness usually characterized by fever, muscle aches, fatigue, and upper respiratory infection and inflammation. Children and young adults usually recover from influenza within 3–7 days with no complications; however, in older adults, especially those over 65 with underlying conditions such as heart disease or lung illnesses, influenza can be deadly. Most of the hospitalizations and deaths from influenza occur in this age group. Although an influenza vaccine is available, it does not confer complete protection against all strains of influenza viruses.

Like all viruses, orthomyxoviruses cause illness by...