Influenza, or flu, is a viral infection of the lungs. There are 2 main types of flu virus, A and B. Each type includes many different strains and new strains emerge periodically. Flu outbreaks occur most often in late fall and winter.
Image of flu virus cell
Flu Symptoms
  • Dry cough
  • Extreme tiredness
  • Fever (usually high)
  • Headache
  • Muscle aches
  • Runny or stuffy nose
  • Sore throat
  • Stomach symptoms (more common in children than adults), such as:
    • Diarrhea
    • Nausea
    • Vomiting
How Flu Spreads
Flu viruses spread mainly from person to person through coughing or sneezing. Sometimes people become infected by touching something with flu viruses on it and then touching their mouth or nose. Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 days after becoming sick. That means that you can pass on the flu to someone else before you know you are sick, as well as while you are sick.

Preventing the Flu: Get Vaccinated
The single best way to prevent the flu is to get a flu vaccination each fall.

View the video from the CDC "Why Vaccination Matters: Personal Stories from Families Affected by Influenza."

Practice Good Respiratory Hygiene
There are things you can do to protect yourself and your family against the spread of the flu and other germs and viruses:
  • Avoid close contact with people who are sick.
  • Avoid touching eyes, nose or mouth.
  • Cover your nose and mouth with a tissue when coughing or sneezing, or cough into your elbow.
  • Stay home when you are sick.
  • Visit the CDC's website for more information.
  • Wash hands thoroughly and often.
Treating the Flu
Antiviral medications can be used to treat and/or prevent influenza A viruses. However, flu strains can become resistant to antiviral medications and there are side effects.

Flu Terms Defined
  • Avian (or bird) flu is caused by influenza viruses that occur naturally among birds. The H5N1 variant, currently spreading throughout the world in birds, is deadly to domestic fowl and can be transmitted from birds to humans. There is no human immunity and no vaccine is available.
  • Pandemic flu is human flu that causes a global outbreak, or pandemic, of illness. Because there is little natural immunity, the disease can spread easily from person to person, and is likely to cause serious illness. Currently, there is not a pandemic of flu.
  • Seasonal (or common) flu is a respiratory illness that can be transmitted person to person. Most people have some immunity, and a vaccine is available.
Difference Between Seasonal Flu & Pandemic Flu
Find out the difference between seasonal flu and pandemic flu at

Learn More
View a glossary of terms and definitions.

Source: Some material/format on pandemic influenza adapted from Sonoma County Health Department