Influenza spreads from person to person mainly through the air. When an infected person coughs, sneezes or even talks, they expel droplets of mucus and saliva containing huge numbers of virus particles. Larger droplets travel about one to two metres before falling, but smaller droplets (aerosols) can stay suspended in the air for longer. A healthy person breathes these in, and the virus reaches the epithelium of the nose, throat and airways, its site of infection.Flu can also spread by indirect contact. Droplets settle on surfaces such as door handles, keyboards and hands. These contaminated objects are called fomites. If someone touches a fomite and then touches their mouth, nose or eyes, the virus can enter the body. The virus can stay infective on hard surfaces for up to about a day.An infected person can pass on the virus from about a day before symptoms appear until around five to seven days after, so people spread flu before they know they are ill.Transmission is higher when:
• people are crowded together indoors, as in schools, care homes and public transport
• the weather is cold and dry, which helps the virus survive in the air (flu peaks in winter in the UK)
• hands are not washed and coughs are not covered.Some influenza strains infect birds and pigs as well as humans. Close contact with infected animals can transfer these strains to people, which is how some new pandemic strains arise.
• people are crowded together indoors, as in schools, care homes and public transport
• the weather is cold and dry, which helps the virus survive in the air (flu peaks in winter in the UK)
• hands are not washed and coughs are not covered.Some influenza strains infect birds and pigs as well as humans. Close contact with infected animals can transfer these strains to people, which is how some new pandemic strains arise.