Influenza (Flu)

Influenza, commonly called "the flu," is an infection of the respiratory tract caused by the influenza virus. Compared with most viral respiratory infections, such as the common cold, influenza infection often causes a more severe illness. Typical influenza illness includes fever (usually 100 degrees F to 103 degrees F in adults and often even higher in children) and respiratory symptoms, such as cough, sore throat, runny or stuffy nose, as well as headache, muscle aches and extreme fatigue. Although nausea, vomiting and diarrhea can sometimes accompany influenza infection, especially in children, these symptoms are rarely the primary symptoms. The term "stomach flu" is a misnomer that is sometimes used to describe gastrointestinal illnesses caused by organisms other than influenza viruses.

Most people who get the flu recover completely in 1 to 2 weeks, but some people develop serious and potentially life-threatening medical complications, such as pneumonia. Over the past decade, influenza and pneumonia have been associated with an average of 3,500 deaths a year in Illinois. Since 1992, the highest number of flu and pneumonia deaths was the 4,021 recorded in 1993. Flu-related complications can occur at any age, but the elderly and people with chronic health problems are much more likely to develop serious complications after influenza infection than are young, healthier people. During most flu seasons, which typically run from October through May, between 10 percent and 20 percent of the population is infected with influenza viruses. More than 200,000 people are hospitalized from flu complications each year in the U.S.

Influenza Viruses

Influenza viruses are divided into three types, designated A, B and C. Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates for hospitalization and death. Influenza type C differs from types A and B in some important ways. Type C infection usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do. Efforts to control the impact of influenza are aimed at types A and B.

While there are many different flu viruses, each season a flu vaccine protects against the three or four viruses that research suggests will be most common. Three kinds of flu viruses commonly circulate among people today: Influenza A (H1N1) viruses, influenza A (H3N2) viruses, and influenza B viruses.

All of the 2016-2017 influenza vaccine is made to protect against the following:

  • A/California/7/2009 (H1N1)-like viruses
  • A/Hong Kong/4801/2014 (H3N2)-like viruses
  • B/Brisbane/60/2008-like viruses. (Victoria lineage)

Some of the 2016-2017 flu vaccine is quadrivalent vaccine, which also protects against an additional lineage of B virus. For this season that will be a B/Phuket/3073/2013-like virus. This is a B/Yamagata lineage virus.

Vaccines that give protection against three kinds of viruses are called trivalent vaccines. Vaccines that give protection against four viruses are called quadrivalent vaccines.


The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death.

Symptoms of flu include:

  • fever (usually high)
  • headache
  • extreme tiredness
  • dry cough
  • sore throat
  • runny or stuffy nose
  • muscle aches
  • Stomach symptoms, such as nausea, vomiting, and diarrhea, also can occur but are more common in children than adults.

While getting a flu vaccine each year is the best way to protect against flu, influenza antiviral drugs can fight against influenza, offering a second line of defense against the flu.

Antiviral Drugs

Antiviral drugs are an important second line of defense against the flu.

  • If you do get the flu, antiviral drugs are an important treatment option. (They are not a substitute for vaccination.)
  • Antiviral drugs are prescription medicines (pills, liquid or an inhaler) that fight against the flu by keeping flu viruses from reproducing in your body.
  • Antiviral drugs can make your illness milder and make you feel better faster. They may also prevent serious flu complications. This could be especially important for people at high risk.
  • For treatment, antiviral drugs work best if started soon after getting sick (within two days of symptoms).

The U.S. Centers for Disease Control and Prevention has issued interim guidance on which antiviral drugs to use during the 2016-2017 flu season. The three antiviral drugs are:

  • Oseltamivir (brand name Tamiflu ®)
  • Zanamivir (brand name Relenza ®)
  • Intravenous Peramivir (brand name Rabivab®)

Antiviral drugs differ in terms of who can take them, how they are given, their dose (which can vary depending on a person’s age or medical conditions), and side effects.

For more information, see CDC Influenza Antiviral Medications: Summary for Clinicians or consult the package insert for each drug. Your doctor can help decide whether you should take an antiviral drug this flu season and which one you should use.

If You Get Sick

Most healthy people recover from the flu without complications. If you get the flu:

  • Stay home from work or school.
  • Get lots of rest, drink plenty of liquids, and avoid using alcohol and tobacco.
  • There are over-the-counter (OTC) medications to relieve the symptoms of the flu (but never give aspirin to children or teenagers who have flu-like symptoms, particularly fever).
  • Remember that serious illness from the flu is more likely in certain groups of people including people 65 years of age and older, pregnant women, people with certain chronic medical conditions and young children.
  • Consult your doctor early on for the best treatment, but also be aware of emergency warning signs that require urgent medical attention.

Emergency Warning Signs

Seek emergency medical care if you or someone you know is having any of following warning signs discussed below.

In children, emergency warning signs that need urgent medical attention include:

  • Fast breathing or trouble breathing
  • Bluish skin color
  • Not drinking enough fluids
  • Not waking up or not interacting
  • Being so irritable that the child does not want to be held
  • Fever with a rash

In adults, emergency warning signs that need urgent medical attention include:

  • Difficulty breathing or shortness of breath
  • Pain or pressure in the chest or abdomen
  • Sudden dizziness
  • Confusion
  • Severe or persistent vomiting

Seek emergency medical care if you or someone you know is experiencing any of the signs above.


The influenza vaccine is provided through local health departments and private health care providers in the state. You should first contact your health care provider regarding availability of the vaccine. If no vaccine is available, then you may contact the local health department that serves the area in which you reside (go to CONTACT US in the top navigation bar). You may also take advantage of the Flu Vaccine Finder located below.