section_path: Epidemiology
section_name: Epidemiology
paragraph_text: Epidemiology
  
  Influenza is typically characterized by seasonal epidemics and sporadic pandemics. Most of the burden of influenza is a result of flu seasons caused by influenza A virus and influenza B virus. Among influenza A virus subtypes, H1N1 and H3N2 circulate in humans and are responsible for seasonal influenza. Cases disproportionately occur in children, but most severe causes are among the elderly, the very young, and the immunocompromised. In a typical year, influenza viruses infect 5–15% of the global population, causing 3–5 million cases of severe illness annually and accounting for 290,000–650,000 deaths each year due to respiratory illness. 5–10% of adults and 20–30% of children contract influenza each year. The reported number of influenza cases is usually much lower than the actual number.
  
  During seasonal epidemics, it is estimated that about 80% of otherwise healthy people who have a cough or sore throat have the flu. Approximately 30–40% of people hospitalized for influenza develop pneumonia, and about 5% of all severe pneumonia cases in hospitals are due to influenza, which is also the most common cause of ARDS in adults. In children, influenza and respiratory syncytial virus are the two most common causes of ARDS. About 3–5% of children each year develop otitis media due to influenza. Adults who develop organ failure from influenza and children who have PIM scores and acute renal failure have higher rates of mortality. During seasonal influenza, mortality is concentrated in the very young and the elderly, whereas during flu pandemics, young adults are often affected at a high rate.
  
  In temperate regions, the number of influenza cases varies from season to season. Lower vitamin D levels, presumably due to less sunlight, lower humidity, lower temperature, and minor changes in virus proteins caused by antigenic drift contribute to annual epidemics that peak during the winter season. In the northern hemisphere, this is from October to May (more narrowly December to April), and in the southern hemisphere, this is from May to October (more narrowly June to September). There are therefore two distinct influenza seasons every year in temperate regions, one in the northern hemisphere and one in the southern hemisphere. In tropical and subtropical regions, seasonality is more complex and appears to be affected by various climatic factors such as minimum temperature, hours of sunshine, maximum rainfall, and high humidity. Influenza may therefore occur year-round in these regions. Influenza epidemics in modern times have the tendency to start in the eastern or southern hemisphere, with Asia being a key reservoir.
  
  Influenza A virus and influenza B virus co-circulate, so have the same patterns of transmission. The seasonality of influenza C virus, however, is poorly understood. Influenza C virus infection is most common in children under the age of two, and by adulthood most people have been exposed to it. Influenza C virus-associated hospitalization most commonly occurs in children under the age of three and is frequently accompanied by co-infection with another virus or a bacterium, which may increase the severity of disease. When considering all hospitalizations for respiratory illness among young children, influenza C virus appears to account for only a small percentage of such cases. Large outbreaks of influenza C virus infection can occur, so incidence varies significantly.
  
  Outbreaks of influenza caused by novel influenza viruses are common. Depending on the level of pre-existing immunity in the population, novel influenza viruses can spread rapidly and cause pandemics with millions of deaths. These pandemics, in contrast to seasonal influenza, are caused by antigenic shifts involving animal influenza viruses. To date, all known flu pandemics have been caused by influenza A viruses, and they follow the same pattern of spreading from an origin point to the rest of the world over the course of multiple waves in a year. Pandemic strains tend to be associated with higher rates of pneumonia in otherwise healthy individuals. Generally after each influenza pandemic, the pandemic strain continues to circulate as the cause of seasonal influenza, replacing prior strains. From 1700 to 1889, influenza pandemics occurred about once every 50–60 years. Since then, pandemics have occurred about once every 10–50 years, so they may be getting more frequent over time.