Influenza, also called "flu," is a viral illness that causes fever, sore throat, muscle aches and cough. Influenza can weaken the body's defenses and lead to complications like bacterial pneumonia. It can also worsen existing chronic health problems like heart disease and diabetes.
The flu season begins in early October and can last as late in the year as May. During an average flu season, 36,000 Americans die and more than 200,000 are hospitalized because of complications from influenza. Some people, such as young children, the elderly, pregnant women and people with certain chronic health conditions are more likely to have complications related to influenza. While most deaths occur among the elderly and people with chronic health conditions, deaths can also occur among younger adults and children.
Influenza: A virus that causes a contagious respiratory illness in a variety of animals and in humans.
Avian Influenza (also known as "bird flu"): Influenza A H5N1 causes mild to severe illness and death in bird populations. It has also been known, rarely, to cause illness in humans. When it is found in humans, it can cause severe respiratory illness and death.
Human Influenza (also known as "the flu" or seasonal influenza): A few strains of the influenza A virus can cause contagious respiratory illness in humans.
Pandemic Influenza: A newly introduced contagious strain of influenza that is found in humans around the world and is associated with an elevated rate of severe illness and death. A pandemic strain can be any influenza strain that is contagious and infectious to humans and for which humans have little or no natural immunity.
The best prevention is to get a flu vaccine each year. Since influenza is primarily spread through respiratory droplets, the following infection prevention measures are also important:
- Cover your mouth and nose with a tissue when sneezing or coughing
- If a tissue is not available, sneeze or cough into your sleeve to contain the droplets (don't sneeze or cough into your hands)
- Clean hands with soap and water or decontaminate hands with an alcohol-based hand rub
- Stay home if you are ill.
For more information on preventing the flu and other diseases, see Infection Prevention Tips for Staying Healthy.
For information on preventing and controlling outbreaks of influenza in institutional settings, please visit Influenza Outbreak Prevention and Control in Long-Term Care Facilities (LTCF).
Novel Influenza A Infections
Novel influenza A virus infection is caused by an influenza A virus subtype that is different from currently circulating human influenza H1 and H3 viruses. Novel subtypes include, but are not limited to, H2, H5, H7, and H9 subtypes. Influenza H1 and H3 subtypes originating from a non-human species or from genetic reassortment between animal and human viruses are also novel subtypes. Novel subtypes are detected with methods available for identification of currently circulating human influenza viruses at state public health laboratories (e.g., real-time reverse transcriptase polymerase chain reaction [RT-PCR]). Confirmation that an influenza A virus represents a novel virus will be performed by CDC's influenza laboratory.
Currently monitored novel influenza subtypes include:
Avian Influenza H5N1 (Bird Flu) Virus
Influenza Related Pages: