Flu is a contagious respiratory illness caused by influenza viruses that infect the
nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at
times can lead to death. The best way to prevent flu is by getting a flu vaccine each year.
Influenza (flu) can cause mild to severe illness, and at times can lead to death.
Flu is different from a cold. Flu usually comes on suddenly. People who have flu often
feel some or all of these symptoms:
fever* or feeling feverish/chills
runny or stuffy nose
muscle or body aches
some people may have vomiting and diarrhea, though this is more common in children
*It’s important to note that not everyone with flu will have a fever.
How Flu Spreads
Most experts believe that flu viruses spread mainly by tiny droplets made when people
with flu cough, sneeze or talk. These droplets can land in the mouths or noses of
people who are nearby. Less often, a person might get flu by touching a surface or
object that has flu virus on it and then touching their own mouth, nose or possibly
How Many People Get Sick with Flu Every Year?
A 2018 CDC study published in Clinical Infectious Diseasesexternal icon looked at the percentage of the U.S. population who were sickened by flu using two
different methods and compared the findings. Both methods had similar findings, which
suggested that on average, about 8% of the U.S. population gets sick from flu each
season, with a range of between 3% and 11%, depending on the season.
Preventing the Flu
The single best way to prevent the flu, even in healthy people, is to get a flu vaccine
each fall. However, you're not helpless when it comes to practicing certain good health
habits that can prevent the flu:
Good health habits include avoiding close contact: If possible, avoid close contact with people who are sick. When you are sick, keep
your distance from others to protect them from getting sick too.
Stay home when you are sick. If possible, stay home from work, school, and errands when you are sick. You will
help prevent others from you catching your illness.
Cover your mouth and nose. If you can't stay home, cover your mouth and nose with a tissue when coughing or
sneezing or catch your cough by coughing into your upper sleeve, not your hands. It
may prevent those around you from getting sick.
Clean your hands. Washing your hands often will help protect you from germs. This measure may be the
single best healthy habit that one can acquire.
Avoid touching your eyes, nose or mouth. Germs are often spread when a person touches something that is contaminated with
germs and then touches his or her eyes, nose, or mouth.
The commonly cited 5% to 20% estimate was based on a study that examined both symptomatic
and asymptomatic influenza illness, which means it also looked at people who may have
had the flu but never knew it because they didn’t have any symptoms. The 3% to 11%
range is an estimate of the proportion of people who have symptomatic flu illness.
The same CID studyexternal icon found that children are most likely to get sick from flu and that people 65 and older
are least likely to get sick from influenza. Median incidence values (or attack rate)
by age group were 9.3% for children 0-17 years, 8.8% for adults 18-64 years, and 3.9%
for adults 65 years and older. This means that children younger than 18 are more than
twice as likely to develop a symptomatic flu infection than adults 65 and older.
Influenza virus infection is so common that the number of people infected each season
can only be estimated. These statistical estimations are based on CDC-measured flu hospitalization rates that are adjusted to produce an estimate of the total number of influenza infections
in the United States for a given flu season.
The estimates for the number of infections are then divided by the census population
to estimate the seasonal incidence (or attack rate) of influenza.
Yes. The proportion of people who get sick from flu varies. A paper published in CIDexternal icon found that between 3% and 11% of the U.S. population gets infected and develops flu
symptoms each year. The 3% estimate is from the 2011-2012 season, which was an H1N1-predominant
season classified as being of low severity. The estimated incidence of flu illness
during two seasons was around 11%; 2012-2013 was an H3N2-predominant season classified
as being of moderate severity, while 2014-2015 was an H3N2 predominant season classified
as being of high severity.
You may be able to spread flu to someone else before you know you are sick, as well
as while you are sick.
People with flu are most contagious in the first 3-4 days after their illness begins.
Some otherwise healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick.
Some people, especially young children and people with weakened immune systems, might
be able to infect others for an even longer time.
Onset of Symptoms: The time from when a person is exposed and infected with flu to when symptoms begin
is about 2 days, but can range from about 1 to 4 days.
Complications of flu can include bacterial pneumonia, ear infections, sinus infections and worsening of
chronic medical conditions, such as congestive heart failure, asthma, or diabetes.
People at High Risk from Flu: Anyone can get flu (even healthy people), and serious problems related to flu can
happen at any age, but some people are at high risk of developing serious flu-related complications if they get sick. This includes people 65 years and older, people of any age with
certain chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant
women, and children younger than 5 years.
The first and most important step in preventing flu is to get a flu vaccine each year. Flu vaccine has been shown to reduce flu related
illnesses and the risk of serious flu complications that can result in hospitalization
or even death. CDC also recommends everyday preventive actions (like staying away
from people who are sick, covering coughs and sneezes and frequent handwashing) to
help slow the spread of germs that cause respiratory (nose, throat, and lungs) illnesses,
It is very difficult to distinguish flu from other viral or bacterial respiratory
illnesses based on symptoms alone. There are tests available to diagnose flu. More
information is available: Diagnosing Flu.