This phenomenon is called antigenic shift. Co-infection of animals such as pigs with different influenza viruses can allow mixing of viral genomes and re-assortment of viral RNA segments thus creating new viruses. New virus strains are derived from two types of events. The possibility that these viruses may mutate and acquire the ability to be efficiently transmitted among human and cause a highly lethal pandemic, remains a serious concern among public health officials. Fortunately these viruses, except in rare instances, cannot be transmitted from humans to other humans. Certain avian influenza viruses such as the H5N1 and H7N9 viruses are highly pathogenic and are associated with a high morality when they infect humans. Influenza A viruses also infects many mammalian species and birds including migratory birds which can spread strains of influenza across the globe. There are no subtypes of influenza B virus, but many different strains exist. For example, an H3N2 influenza type A virus isolated from the eighth individual from whom a clinical sample was obtained in Puerto Rico in 1934 is designated as A/PR/8/34 (H3N2). Influenza virus strains are also identified by the location where they were isolated, the isolate number and the year of isolation. H3N2 and H1N1 are the most common influenza virus subtypes that cause seasonal influenza. Influenza type A strains that infect humans usually have one of three HA proteins (H1, H2, H3) and one of 2 NA proteins (N1, N2). Influenza A viruses are classified into subtypes based on combinations of different HA and NA proteins. The HA protein is involved in entry of the virus into cells and is the primary antigen involved in the immune response to infection or vaccination. Two proteins, the viral hemagglutinin protein (HA) and neuraminidase (NA) protein project outward from the lipid envelope. The virus structure consists of a lipid envelope and a layer of matrix protein that surround the ribonucleoprotein complex. The viral genome consists of 8 unique viral RNA segments that are associated with the NP in a ribonucleoprotein complex.

The influenza viruses are RNA viruses from the family Orthomyxoviridae and are classified as either type A or type B based upon the viral nucleoprotein (NP). Signs seen on physical examination include conjunctivitis, nasal discharge, pharyngitis without exudate, cervical adenopathy and rarely rales. Unlike adults, children may present with vomiting and diarrhea. Some symptoms, especially cough and malaise, may persist for up to 2 weeks. The onset of flu symptoms is usually abrupt and characterized by fever (up to 104☏), chills, myalgia, headache, lumbosacral backache, weakness, sore throat, and a dry cough. Children and individuals with a compromised immune system may be contagious for up to 2 weeks. Adults 'shed' influenza virus for approximately 7 days following infection during which time they can spread the infection and thus considered contagious. The time from infection until occurrence of symptoms (incubation period) is usually 1-3 days.

Influenza virus is transmitted via aerosols and infects cells of the respiratory tract. Children are susceptible to complications such as inner ear infections (otitis media), viral pneumonia, secondary bacterial pneumonia, and Reye's syndrome, a potentially fatal complication that can be precipitated by aspirin therapy. Other complications of influenza infection include encephalitis and meningitis. Influenza infection causes damage to cells of the respiratory tract tissue which predisposes infected individuals to a secondary bacterial infection.

The elderly, patients with heart and lung disease, and immunocompromised patients (cancer patients, those with HIV infection, transplant patients, etc.) are particularly susceptible to influenza. In healthy adults, bronchitis and pneumonia (primary viral and secondary bacterial) are the most common complications of influenza. 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. Influenza is a viral infection with a characteristic constellation of symptoms, including chills, fever, headache, fatigue, muscle aches, cough, and runny nose, often referred to as the “flu.' Influenza occurs primarily during the winter months (November through April in the United States).

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