Influenza

Influenza (The Flu) is one of the most prevalent and serious respiratory infections in the United States and the world, leading to 40,000 deaths annually in the US alone. Occasional outbreaks of particularly virulent strains, such as the 2009 H1N1 "swine flu" pandemic and the "bird flu" epidemic, are cause for headline news stories and world-wide vaccination campaigns. The 2009 H1N1 pandemic is thought to have infected ~ 22 million Americans, leading to 98,000 hospitalizations and 3,9000 deaths (estimates by CDC, see link below), while the H5N1 avian influenza epidemic, mostly in Asia, has resulted in 400 human cases and a 60% mortality.

Summary 

• Cause: Influenza virus (Orthomyxovirus) - strains designated A, B, C and by HA and NA antigens
• Transmission: airborne/respiratory droplets; direct contact with infected animals (some strains)
• Reservoirs include humans, poultry (chickens, ducks etc), pigs
• Symptoms: fever, malaise, myalgia, pharyngitis, cough, otitis media, GI upset
• Complications: pneumonia, carditis, encephalitis (Reye's syndrome)
• Pathogenesis: Primary infection of mucoepithelium causes cytopathic effect, local inflammation and loss of epithelial cells, esp. ciliated/mucus-producing cells. Systemic symptoms are due to interferons.
• Treatment: Supportive; amantadine (uncoating inhibitor), oseltamivir (NA inhibitor)
• Immunity: intermediate
• Vaccine: annual vaccine needed - effective only against some strains

References

• http://www.cdc.gov/h1n1flu/estimates/April_October_17.htm