What Is Influenza A

There are three Influenza viruses that infect humans which include influenza A, influenza B, and influenza C. However, the influenza A virus is more commonly seen. The influenza A virus has been attributed to cause serious epidemics and pandemics.

The influenza A virus infects a wide array of species including pigs, horses, birds, humans, and other animals. Although there are many similarities between these influenza A viruses, they are by no means the same. Each influenza A virus is genetically tailored to infect that species.

Influenza is a respiratory infection caused by an influenza virus. Many refer to influenza as the “flu”. The influenza virus may cause mild or severe illness which can lead to death.

The influenza virus In the U.S. is estimated annually to:

  • Infect 5-20% of the population
  • Account for 200,000 hospitalizations
  • Account for 36,000 deaths

What is Influenza?

Influenza is a respiratory tract infection involving the upper airways and lungs. Influenza is highly contagious, transmitted by saliva and airborne droplets expelled by coughing, and sneezing. The incubation period is very short, ranging from one to three days.

Common symptoms of influenza include:

  • Fever
  • Chills
  • Muscle aches and pains
  • Dry cough
  • Nasal congestion
  • Sore throat
  • Headache
  • Malaise
  • Fatigue
  • Sneezing
  • Sweating
  • Irritated eyes
  • Runny or stuffy nose

Other symptoms that are more common in children than adults include:

  • Nausea
  • Vomiting
  • Diarrhea

Many individuals may experience complications from the flu. Complications that can occur include:

  • Dehydration
  • Bacterial pneumonia
  • Worsening of chronic medical conditions

Children may experience the above complications along with the following:

  • Sinus problems
  • Ear infections

The influenza virus is spread from human to human when infected droplets are expelled by sneezing or coughing. Individuals may also become infected by touching a contaminated surface or object then their nose or mouth. Healthy adults may infect others one day prior to the development of symptoms up to five days after the onset of symptoms.

The best way to protect against the influenza virus is to get an annual influenza vaccination. Individuals who wish to reduce their chance of becoming infected should get vaccinated. However, certain individuals at a higher risk of influenza infection are strongly encouraged and include:

  • Individuals 50 years or older
  • Children 6-23 months of age
  • Women pregnant during the flu season
  • Individuals with chronic conditions of the heart or lungs
  • Individuals with diabetes
  • Individuals with a weakened immune system
  • Health care workers and caregivers

Certain diseases can weaken the body’s immune system which includes:

  • HIV patients
  • Cancer patients
  • Organ transplant patients
  • Patients receiving chemotherapy
  • Patients receiving radiation therapy
  • Patients receiving steroid therapy

Certain individuals may not be ideal candidates for the influenza vaccination. Individuals who should not be vaccinated include:

  • Individuals with an allergy to chicken eggs
  • Children under the age of 6 months
  • Individuals currently ill exhibiting a fever
  • Individuals who have had a severe reaction to an influenza vaccine
  • Individuals who developed Guillain-Barre Syndrome 6 weeks after vaccination

Influenza vaccines are developed each year based off the predominant strains in the Southern Hemisphere. Protection provided by the vaccine depends on effectiveness of the strain used to make the vaccine. Vaccinations typically begin in October.


  1. CDC
  2. National Foundation for Infectious Diseases

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