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Influenza (Flu)

Read a FAQ from the Centers for Disease Control about the 2017-18 flu season »

What is influenza?

Influenza is a viral infection of the nose, throat, bronchial tubes and lungs. There are two main types of virus: influenza A and influenza B. Each type includes many different strains, which tend to change each year. Influenza is not a reportable disease in New York City, although nosocomial (hospital-acquired) outbreaks are reportable to the New York State Department of Health.

When does influenza occur?

Influenza occurs most often in the fall and winter months. In the United States, seasonal flu activity can begin to increase in the early winter but in past years has not reached peak levels until late December through early March. Illnesses resembling influenza may occur in the summer months but they are usually due to other viruses.

Who gets influenza?

Anyone can get influenza, but it is most serious in the elderly, in young children, in people with chronic underlying illnesses (such as cancer, emphysema or diabetes), or those with weak immune systems.

How is influenza spread?

Influenza is highly contagious and is easily transmitted through contact with droplets from the nose and throat of an infected person during coughing and sneezing.

What are the symptoms of influenza?

Typical symptoms include headache, fever, chills, cough, sore throat and body aches. Intestinal symptoms, such as vomiting or diarrhea, are uncommon. Although most people are ill for only a few days, some people have a much more serious illness, such as pneumonia and may need to be hospitalized. Approximately 36,000 people die each year in the United States from influenza or related complications; in 2002 in New York City alone over 2,500 people died.

How soon after infection do symptoms appear?

Influenza generally occurs within 1 to 5 days after exposure.

When and for how long is a person able to spread influenza?

The contagious period varies, but probably begins the day before symptoms appear and lasts up to a week.

How is influenza diagnosed?

Usually, a doctor will diagnose influenza based on typical symptoms of fever, chills, headache, cough, and body aches. When indicated, there are specific lab tests that can be done to confirm the diagnosis of influenza. There are several rapid tests that can detect influenza in a matter of hours, but as they vary in accuracy; specimens with negative results should also be tested using viral culture.

Does past infection with influenza make a person immune?

Generally, no. The viruses that cause influenza frequently change, so people who have been infected or given an influenza shot in previous years may become infected with a new strain. Because of this, and because any immunity produced by the shot decreases in the year after vaccination, people need to be vaccinated every year.

Who are the high-risk groups?

The following groups are at an increased risk for serious illness with influenza and should receive vaccine:

  • Adults 50 years of age and older, particularly those age 65 and over;
  • Children 6-23 months of age;
  • People age 6 months and older with chronic medical conditions, including heart disease, pulmonary disorders (including asthma), diabetes, kidney disease, hemoglobinopathies, and compromised immune systems (HIV or immunosuppressive therapy);
  • Women who will be pregnant (at any gestational age) during the influenza season;
  • Residents of nursing homes and chronic-care facilities;
  • Children and adolescents, age 6 months to 18 years, receiving long-term aspirin therapy;

Additional individuals who can transmit influenza to high-risk individuals should also receive influenza vaccine:

  • Health care workers involved in direct patient care;
  • Out-of-home caregivers and household contacts of persons in high-risk groups (e.g. persons > 65 years; persons with chronic conditions such as diabetes, heart or lung disease or weakened immune systems because of illness or medication; and children aged < 2 years).

What is the treatment for influenza?

Rest and liquids are usually adequate. There are four prescription drugs available to prevent or reduce the severity of influenza infection. Two of these drugs, amantadine and rimantidine, are effective against influenza type A, but are not effective against type B. Zanamivir and oseltamivir may reduce the duration of both influenza A and B. Most people do not require treatment with these medications but your physician can determine if you should receive one of these antiviral agents.

How can influenza be prevented?

Routine immunization against influenza is the most important control measure. Influenza vaccines (flu shots) are available through your personal physician or the health department. Because new influenza viruses often appear every year, the effectiveness of the vaccine sometimes varies from one year to the next. Nevertheless, studies have shown that even in years when new strains emerge, people in high-risk groups who obtain annual flu shots tend to have a milder illness and are less likely to be hospitalized.

For additional information, call the NYC Department of Health's Flu Vaccination Information Line at 311.