Influenza

Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus. There are four types of influenza virus, types and B (seasonal), C which is less frequent causing mild infections and not a public health threat, and D which primarily infects cattle and is not known to infect or cause illness in people.

Seasonal influenza is characterized by a sudden onset fever or feeling feverish/chills, a cough (usually dry), headache, muscle pain and joint pain, severe malaise (feeling unwell), fatigue, sore throat and runny or stuffy nose. Vomiting and diarrhea is more common in children than adults.

Influenza A
Influenza A is a major subtype of flu viruses, there are 18 known HA and 11 NA subtypes of influenza A, although of those that can infect humans only H1N1 and H3N2 are currently in wide circulation.

Avian flu
Avian influenza viruses are influenza A viruses, and infect birds including chickens, ducks and wild birds but rarely infect people. Influenza A virus subtypes H5, H7 and H9 are the most likely to infect humans, particularly H5N1 and H7N9. H5N1 has infected people in 16 different countries, and kills around 50% of people it infects.

Swine flu
Swine flu is the subtype of influenza A known as H1N1.

Influenza B
The other main type of flu is influenza B.

Influenza C
Influenza C is less common than influenza A and B.

Influenza D
Influenza D is less common than influenza A or influenza B.

Epidemiology
All age groups can be infected with flu but there are groups that are at more risk than others.
 * children under 59 months
 * health care workers
 * elderly
 * pregnant women
 * individuals with chronic medical conditions


 * asthma
 * cardiac
 * Diabetes
 * hematologic
 * liver
 * metabolic
 * neurodevelopmental
 * pulmonary
 * renal (kidney related)

Influenza spreads easily with rapid transmission in crowded areas such as schools and nursing homes. Sneezing and coughing spreads droplets containing viruses which can spread over three feet. Persons within range can become infected by breathing in these droplets. The virus also spreads by hands contaminated with influenza. From time of infection to illness is about 2 days but can be one to four days.
 * individuals with immunosuppressive conditions
 * HIV/AIDS
 * malignancy (cancer)
 * receiving chemotherapy or steroids

Learn more

 * Flu Symptoms - What are the emergency warning signs of flu sickness? (Adults, children, and infants.)
 * Influenza Video short
 * Influenza (Seasonal) Fact Sheet - World Health Organization
 * Influenza A subtypes - CDC
 * H5N1 - CDC