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Flu (Influenza)

What is the flu?

The flu is a common infectious disease caused by influenza (flu) viruses. The flu usually affects a person's breathing system. Seasonal flu generally occurs in the fall and winter and is caused by human influenza viruses.



What about H1N1 swine flu?

H1N1 swine flu is an influenza virus that was found in April 2009.  It caused illness in people worldwide. A flu pandemic due to H1N1 swine flu was declared in June 2009.  We expect that H1N1 will still be around this year along with other flu viruses.  This is why it is important for everyone 6 months and older to get a flu shot.


How does the flu spread?

The flu spreads through tiny wet drops produced when a person coughs, sneezes, or talks. A person can get the flu by breathing in these wet drops, or by touching items and surfaces covered with these drops and then touching their mouth, nose, or eyes.


When can someone spread the flu to others?

People who have the flu may spread it to others about 1 day before getting sick to 5 -7 days after. Children and people with weak immune systems can spread the flu virus for a longer period of time. However, people are most contagious during the first three days of illness.


How can I protect myself from the flu?

Use healthy habits:



Is there a vaccine?

Yes. It is important for everyone 6 months and older to get a flu shot this fall and winter. There are two types of flu vaccine available this year. One type protects against three types (strains) of flu, the other protects against four strains of flu virus. Contact your health care provider for updates on where you can get the vaccine. If you do not have a doctor, click here to find out where to go in San Francisco.


Do I need to wear a mask?

People who feel sick may be asked by their doctor to wear a mask while in the doctor's office or clinic. People who are sick may choose to wear a mask when around others.


What are signs and symptoms of the flu?

The usual signs and symptoms of the flu are cough, sore throat, runny or stuffy nose, body aches, headache, chills, and feeling very tired. Most people also have a fever. Others may throw-up and have diarrhea.


Is it a cold or the flu?

In general, the flu is worse than the common cold. Colds are usually milder than the flu.






Usually 102 degrees F, but can go up to 104 degrees F and usually lasts 3 to 4 days.

Rare in adults and older children, but can be as high as 102 degrees F in infants and small children.

Headache Sudden onset and can be severe Rare
Muscle Aches Usually, and often severe None or mild
Tiredness/Weakness Can last 2 or more weeks Mild
Extreme Exhaustion Sudden onset and can be severe Never
Runny Nose Sometimes Often
Sneezing Sometimes Often
Sore Throat Sometimes Often
Cough Usually, and can become severe Mild to moderate

How serious is the flu?

The flu does not usually cause serious problems. Occasionally it can cause severe disease. Some people have had to go to the hospital and a small number of people have died. Historically, seasonal flu causes between 3,000 to 49,000 deaths in the United States each year.


Who is more likely to get very sick with the flu?

Below is a list of people who may get very sick if they have the flu.  It is important that they, and the people around them, get a flu shot.   


  • People with lung disease like asthma
  • People of all ages with ongoing medical conditions like diabetes, heart disease, kidney or liver disease, blood cell disease including sickle cell, or neurological diseases that affect swallowing or breathing
  • Pregnant women and women who have given birth within 2 weeks
  • Children younger than 5 years, especially those younger than 2 years
  • Adults age 50 years and over
  • People with weak immune systems (due to disease or medicines)


What should I do if I am sick with a flu-like illness?

We recommend that you stay home and stay away from others until 24 hours after your fever is gone (without the use of fever-reducing medicine like acetaminophen (e.g., Tylenol) or ibuprofen (e.g., Motrin, Advil).  Click here for more guidance on what to do if you are sick and when to contact your health care provider.


Additional information

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