Influenza viruses pass from person to person mainly in droplets expelled during sneezes and coughs. When a person breathes in virus-laden droplets, the hemagglutinin on the surface of the virus binds to enzymes in the mucous membranes that line the respiratory tract. The enzymes, known as proteases, cut the hemagglutinin in two, which enables the virus to gain entry into cells and begin to multiply. These proteases are common in the respiratory and digestive tracts but not elsewhere, which is why the flu causes primarily a respiratory illness with occasional gastrointestinal symptoms. In the 1990s scientists discovered that some flu strains also can use the enzyme plasmin to cut hemagglutinin. Plasmin is common throughout the body, enabling the flu strains to infect a variety of tissues.
Although an influenza epidemic can occur at any time of year, flu season in temperate regions typically begins with the approach of winter—November in the Northern Hemisphere, April in the Southern Hemisphere. Flu viruses spread more easily during cold weather because people tend to spend more time crowded together in homes and schools, as well as buses, subways, and other places with poor ventilation. An epidemic may be restricted to a town or city or may quickly spread geographically as infected people travel aboard motor vehicles, airplanes, and ships.
Although an influenza epidemic can occur at any time of year, flu season in temperate regions typically begins with the approach of winter—November in the Northern Hemisphere, April in the Southern Hemisphere. Flu viruses spread more easily during cold weather because people tend to spend more time crowded together in homes and schools, as well as buses, subways, and other places with poor ventilation. An epidemic may be restricted to a town or city or may quickly spread geographically as infected people travel aboard motor vehicles, airplanes, and ships.
