Seasonal influenza is commonly referred to as “the flu.” It is a contagious respiratory illness caused by viruses that infect the nose, throat, and lungs. There are two main types of influenza viruses: A and B. Each type includes many different strains that tend to change from year to year. The flu spreads mainly from person to person by droplets from the nose or throat that are released when an infected person coughs or sneezes. The best way to prevent the flu is to get vaccinated every year.
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There are a lot of possible subtypes of influenza A that are not a part of the seasonal flu. All of these subtypes are considered novel because they don’t regularly infect humans. Instead, strains like H7N9 and H3N2v can occur in animals like poultry, pigs, cats, dogs, and even horses. When a virus jumps from an animal to a human, that’s called a spillover event. While rare, most spillover events in the U.S. have been with variant influenza, or a strain known to exist in pigs.
Most novel flu viruses do not spread easily from one human to another. If any flu virus develops this ability, however, it could potentially lead to a pandemic – a large-scale global epidemic. Public health monitors all novel flu viruses to help prevent pandemics.