Seasonal influenza, commonly referred to as the flu, is an infectious disease. In the United States, flu season usually occurs between October and May. The influenza virus has the ability to change easily; however, there is usually enough similarity in the virus from one year to the next that the general population is partially immune from previous infection or vaccination. People may be contagious for 1-2 days before symptoms until 4-5 days after onset; young children can be infectious for 10 or more days.
Pandemic influenza occurs when a new subtype of influenza virus develops and there is little or no immunity (protection due to previous infection or vaccination), causes serious illness and is easily passed from person to person.
Signs and Symptoms
Signs and Symptoms of influenza may include:
Sudden fever higher than 100.4°F (38°C)
Muscle aches or pain
Feeling of weakness, tiredness and/or exhaustion
Cough or sore throat
Runny or stuffy nose
Signs of dehydration such as decreased urine, dry mouth & eyes, dizziness, etc.
Diarrhea, vomiting, abdominal pain (more common in children)
How influenza spreads
Flu viruses can be spread by being near someone who is sick and coughing or sneezing. People may become infected by touching surfaces that an infected person has touched or may have sneezed or coughed on.