Influenza (flu) is a highly contagious virus that infects the respiratory tract and causes a cascade of symptoms known as "the flu". In the United States, flu season typically runs from November through April, with the largest number of cases seen between late December and early March. According to the United States Centers for Disease Control and Prevention (CDC), influenza infections cause more than 200,000 hospitalizations and 36,000 deaths annually. In healthy adults, bronchitis and pneumonia (primary viral and secondary bacterial) are the most common complications of influenza. Together, influenza and pneumonia are the 6th leading cause of death in the United States. Influenza may be particularly fatal in the elderly and immunocompromised (cancer patients, those with HIV infection, individuals receiving steroids, chemotherapy, or radiation therapy). Influenza infection compromises the mucociliary lining of the respiratory tract and makes individuals susceptible to such bacterial agents as Staphylococcus aureus, Streptococcus pneumonia, and Hemophilus influenza. Other complications of influenza infection include encephalitis and meningitis. Otitis media, croup, primary viral pneumonia, and secondary bacterial pneumonia are influenza complications found in children. Reye's Syndrome is another potential complication from influenza infection that is found in children between the ages of 6 and 11 on long-term aspirin therapy.

Influenza replication takes place within the host’s respiratory epithelial cells. The incubation period lasts 1-3 days, during which time the virus spreads to nearby columnar cells of the respiratory epithelium. After 1-3 days there is usually enough influenza virus present to cause the clinical symptoms associated with influenza infection. Adults will "shed" influenza virus for approximately 7 days following infection, during which time they are considered contagious and capable of spreading the infection. Children and individuals with a compromised immune system may "shed" virus for up to 2 weeks, during which time they remain contagious. Infected respiratory epithelium demonstrates significant necrosis which predisposes the infected individual to a secondary bacterial infection.

The onset of flu symptoms is abrupt and characterized by fever (up to 104° F), chills, myalgia, headache, lumbosacral backache, weakness, pharyngitis, and a dry cough. Fever usually lasts 2-4 days. The other symptoms, especially cough and malaise, may persist for up to 2 weeks. Unlike adults, children may present with vomiting and diarrhea. Physical examination may demonstrate evidence of conjunctivitis, nasal discharge, hyperemic pharyngeal mucosa without exudate, cervical adenopathy, and, rarely, rales.

The influenza viruses are RNA viruses from the family Orthomyxoviridae and are identified as either type A or type B based upon the immunoreactivity of the viral nucleoprotein (NP). The nucleoprotein is associated with the 8 unique viral RNA segments. Together the NP and RNA form the viral ribonucleoprotein. A lipid envelope surrounds the internal ribonucleoprotein and contains the viral hemagglutinin protein (HA) and neuraminidase (NA) protein antigens that project outward from the lipid envelope. The human influenza type A strains are identified by the presence of the hemagglutinin protein (H1, H2, H3) and by the presence of the neuraminidase protein (N1, N2). The influenza virus A strain is thus identified, for example, as an H1N2 virus. The influenza virus strains are also identified by the location where they were isolated, the isolate number and the year of isolation. For example, an H1N3 influenza type A virus isolated from the eighth individual from whom a clinical sample was obtained in Puerto Rico in 1934 would be identified as A/PR/8/34 (H1N3).

Vaccination is the primary means to protect an individual from influenza and help prevent the spread of disease. The inactivated vaccine is highly purified and 70-90% effective in preventing influenza in healthy adults. In addition, vigilance in handwashing and avoiding close contact with known infected individuals are also important in preventing the spread of influenza. The Advisory Committee on Immunization Practices (ACIP) has identified specific "high risk" individuals who should receive the vaccine. These individuals have the greatest potential of developing severe disease or complications:

  • Children aged 6-23 months
  • People aged 50 years and older
  • People with chronic heart, lung, or kidney disease, diabetes, immunosuppression, or severe forms of anemia
  • Residents of nursing homes or other chronic care facilities
  • Women who will be over 3 months pregnant during the influenza season
  • Children and teens receiving long-term aspirin therapy