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Types of Influenza Viruses

Influenza A virus Panel (H1 and H3) Flu Symptoms & Diagnosis

Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1 through H18 and N1 through N11, respectively). While more than 130 influenza A subtype combinations have been identified in nature, primarily from wild birds, there are potentially many more influenza A subtype combinations given the propensity for virus “reassortment.” Reassortment is a process by which influenza viruses swap gene segments. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information. Current subtypes of influenza A viruses that routinely circulate in people include A(H1N1) and A(H3N2). Influenza A subtypes can be further broken down into different genetic “clades” and “sub-clades.” See the “Influenza Viruses” graphic below for a visual depiction of these classifications. Influenza (also known as “flu”) is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. This page provides resources about flu symptoms, complications, and diagnosis.

  • Fever*/feeling feverish or chills
  • Cough
  • Sore throat
  • Runny or stuffy nose
  • Muscle or body aches
  • Headaches
  • Fatigue (tiredness)
  • Some people may have vomiting and diarrhea, though this is more common in children than adults.
Information for Specific Higher Risk Groups

People at Higher Risk of Flu Complications

Following is a list of all the health and age factors that are known to increase a person’s risk of getting serious flu complications: Adults 65 years and older Children younger than 2 years old1 Asthma Neurologic and neurodevelopment conditions Blood disorders (such as sickle cell disease) Chronic lung disease (such as chronic obstructive pulmonary disease [COPD] and cystic fibrosis) Endocrine disorders (such as diabetes mellitus) Heart disease (such as congenital heart disease, congestive heart failure and coronary artery disease) Kidney diseases Liver disorders Metabolic disorders (such as inherited metabolic disorders and mitochondrial disorders) People with a body mass index (BMI) of 40 kg/m2 or higher People younger than 19 years old on long-term aspirin- or salicylate-containing medications. People with a weakened immune system due to disease (such as people with HIV or AIDS, or some cancers such as leukemia) or medications (such as those receiving chemotherapy or radiation treatment for cancer, or persons with chronic conditions requiring chronic corticosteroids or other drugs that suppress the immune system) People who have had a stroke Other people at higher risk from flu: Pregnant people and people up to 2 weeks after the end of pregnancy People who live in nursing homes and other long-term care facilities People from certain racial and ethnic minority groups are at increased risk for hospitalization with flu, including non-Hispanic Black persons, Hispanic or Latino persons, and American Indian or Alaska Native persons 1 Although all children younger than 5 years old are considered at higher risk of serious flu complications, the highest risk is for those younger than 2 years old, with the highest hospitalization and death rates among infants younger than 6 months old.

Flu Treatment

If you get sick with flu, influenza antiviral drugs may be a treatment option. Antiviral drugs work best when started early, ideally no later than two days after your flu symptoms begin. Check with your doctor promptly if you are at higher risk of serious flu complications and you get flu symptoms. People at higher risk of flu complications include young children, adults 65 years of age and older, pregnant people, and people with certain medical conditions such as asthma, diabetes and heart disease. Treatment of flu with influenza antiviral medications works best when started within two days after flu symptoms begin and can lessen symptoms and shorten the time you are sick by about a day. Starting antiviral treatment shortly after symptoms begin also can help reduce some flu complications.

Prevent Seasonal Flu
How Flu Spreads
How Many People Get Sick with Flu Every Year?
Who is most likely to get sick with flu?
How is seasonal incidence of flu estimated?
Does seasonal incidence of flu change based on the severity of flu season?
Period of Contagiousness
Complications of Flu
Preventing Seasonal Flu
Treating Flu
What are the benefits of antiviral drugs?
When should antiviral drugs be taken for treatment?
What antiviral drugs are recommended this flu season?