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What Is the Difference Between Influenza A & B?

By Marcia Frost ; Updated July 27, 2017

Influenza was first diagnosed in the 1930s and has since been classified into three types: A, B and C. Influenza C has never caused a major epidemic, but A and B each have. Also known as the flu, influenza is a highly contagious virus that can have severe complications. The two strands have different biological mechanisms but are largely similar, especially in the symptoms.


You catch influenza A or B when you breathe in the virus from someone else's cough or sneeze, or when you touch a surface with the virus on it (such as a phone or faucet) and then your nose, mouth or eyes. The risk of infection from both influenza A and influenza B is higher in crowded public areas and living quarters.


Influenza A is more likely to appear between early winter and the beginning of spring in areas that have distinct seasons. Influenza B can appear any time of the year. Both types begin to show symptoms within a week of exposure to the virus.


The symptoms of influenza A and B are the same: fever; fatigue; body and muscle aches; nausea and vomiting; headache; chills; dizziness; and sore throat. Just as the initial symptoms appear to get better, respiratory problems (difficulty breathing, coughing, runny nose) begin to appear.


A doctor gives you a physical exam to evaluate your signs and symptoms. An antigen detection test, done with a swab of your throat and nose, is the most common way of diagnosing influenza A and B. Blood counts and cultures, as well as sputum cultures, are sometimes completed. If your respiratory symptoms are severe, you may get a chest X-ray.


The symptoms of influenza A are usually more severe than those of influenza B. The most serious epidemics in history have been of influenza A; however, severe symptoms are possible with both viruses.


The most common complication is bacterial pneumonia, a potentially life-threatening condition that causes difficulty and painful breathing, as well as a high fever. Encephalitis, an infection of the brain, can occur as a result of influenza A or B. In children, another possible serious complication from influenza A or B is Reye's syndrome, which affects the nerves and has been linked to using aspirin during a flu outbreak. Reye's syndrome causes nausea and vomiting. It can also lead to confusion and delirium.

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