<unk>  mortality in symptomatic cases in the US for the 2018/2019 season.<unk>  is typically characterized by seasonal epidemics and sporadic pandemics. Most of the burden of<unk>  is a result of flu seasons caused by IAV and IBV. Among IAV subtypes, H1N1 and H3N2 currently circulate in humans and are responsible for seasonal<unk> . Cases disproportionately occur in children, but most severe causes are among the elderly, the very young, and the immunocompromised. In a typical year,<unk>  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<unk>  each year. The reported number of<unk>  cases is usually much lower than the actual number of cases. 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<unk>  develop pneumonia, and about 5% of all severe pneumonia cases in hospitals are due to<unk> , which is also the most common cause of ARDS in adults. In children,<unk>  is one of the two most common causes of ARDS, the other being the respiratory syncytial virus. About 3–5% of children each year develop otitis media due to<unk> . Adults who develop organ failure from<unk>  and children who have PIM scores and acute renal failure have higher rates of mortality. During seasonal<unk> , mortality is concentrated in the very young and the elderly, whereas during flu pandemics, young adults are often affected at a high rate. Seasonal risk areas for<unk> : November–April (blue), April–November (red), and year-round (yellow) In temperate regions, the number of<unk>  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<unk>  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.<unk>  may therefore occur year-round in these regions.<unk>  epidemics in modern times have the tendency to start in the eastern or southern hemisphere, with Asia being a key reservoir of<unk>  viruses. IAV and IBV co-circulate, so the two have the same patterns of transmission. The seasonality of ICV, however, is poorly understood. ICV infection is most common in children under the age of 2, and by adulthood most people have been exposed to it. ICV-associated hospitalization most commonly occurs in children under the age of 3 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, ICV appears to account for only a small percentage of such cases. Large outbreaks of ICV infection can occur, so incidence varies significantly. Outbreaks of<unk>  caused by novel<unk>  viruses are common. Depending on the level of pre-existing immunity in the population, novel<unk>  viruses can spread rapidly and cause pandemics with millions of deaths. These pandemics, in contrast to seasonal<unk> , are caused by antigenic shifts involving animal<unk>  viruses. To date, all known flu pandemics have been caused by IAVs, 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<unk>  pandemic, the pandemic strain continues to circulate as the cause of seasonal<unk> , replacing prior strains. From 1700 to 1889,<unk>  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.
Influenza
Q fever
Swine influenza
Spanish flu
Bird Flu
London flu
Freshers' flu
Polymer fume fever