About five to 20 percent of the US population gets the flu every year.
The CDC estimates that there have been 9 million to 41 million flu illnesses a year from 2010 to 2020.
While getting vaccinated is the most effective way of avoiding the flu, understanding how the flu spreads is a big step towards prevention.
Most commonly, the flu spreads through the air, but it can also spread by physical contact.
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How the flu spreads through the air
The flu is a contagious respiratory illness caused by an influenza virus. You can catch it by simply breathing in the air from people who are infected.
When a person who has the flu coughs, sneezes or even talks, the flu virus travels through the air in droplets. The virus can then enter the mouth or nose of a nearby person and infect their respiratory tract — nose, throat and lungs.
People with the flu can spread it to others up to 6 feet away.
Respiratory viruses can spread through the air in two ways
Droplets are tiny beads of saliva that you produce when you sneeze, cough or talk.
Droplets can travel up to six feet through the air and enter people’s mouths or noses.
Aerosols are tiny respiratory particles produced by breathing or talking.
Because they are so small, aerosols can travel further than droplets and remain suspended in the air for hours.
While most experts believe the flu virus is spread by droplets, newer studies suggest that it might also be spread by aerosols.
How rapidly it spreads depends on humidity, temperature and how crowded a room is.
Most experts, including the CDC, believe that the flu virus is mainly spread by droplets.
How flu spreads by contact
While it’s less common, you can also catch the flu by physical contact.
A person with the flu can leave contagious droplets or particles on surfaces and objects they have been in contact with. After touching a surface with the flu virus, you can transmit it to yourself by touching your nose, mouth or eyes.
How long can a flu virus survive?
24 to 48 hours on hard surfaces, such as plastic or stainless steel.
8 to 12 hours on cloth, paper and tissues.
When is flu most contagious?
Did you know you can pass the flu to another person before even knowing you’re sick? Once exposed to the flu, it can take days before you develop any symptoms, and you can be contagious one day before symptoms begin.
Questions and Answers
Usually, two days after the virus enters the body (sometimes one to four days).
Even if you don’t feel sick, you can still pass the flu to another person. Adults may be able to infect others one day before symptoms develop.
Three to four days after symptoms begin.
Adults may be able to infect other people beginning one day before symptoms develop and up to five to seven days after becoming sick. (Young children and people with weakened immune systems might be contagious for a longer time.)
While you can catch the flu any time of the year, flu activity tends to increase beginning in October and can continue through May. In general, flu activity peaks from December to February. It’s a good idea to get vaccinated earlier in the season to prevent spread.
If you have the flu...
Make sure to limit your contact with friends, family and others, especially people in your household who are not sick. If you need to be around other people, consider wearing a mask.
Stay away from other people until the fever has subsided for over 24 hours without the help of fever-reducing medications. (Keep in mind some people with the flu experience no fever at all.)
How Can You Prevent the Flu?
Overview of Flu Vaccine Types