Influenza ("the flu") is a class of viruses that seasonally infect people and animals in many different forms. Influenza is generally only life threatening in elderly, very young, or in people with compromised immune systems.
Sites that are selling products relating to Influenza belong in Shopping:Health. Sites that are specific to the 1918 Pandemic, Avian Flu (bird flu), Swine Flu (H1N1) or Feline Flu (cat flu) belong in those subcategories.

Influenza A virus subtype H1N1 is the most common cause of flu in humans.  Some strains of H1N1 are endemic in humans, including the strain responsible for the 1918 flu pandemic which killed 50–100 million people worldwide.  Other strains are endemic in pigs and in birds.
A new H1N1 virus strain is responsible for the outbreak first detected in Mexico in March 2009. It appears to be a reassortment of 4 strains, 1 endemic in humans, 1 in birds and 2 in pigs. Although called swine flu due to it predominantly containing swine strains, the World Organisation for Animal Health has proposed the name North American influenza because this new strain in its entirety has not been found in swine.  This strain was formally declared as a global flu pandemic by WHO after an emergency meeting on 11th June 2009.

This category is for sites about avian or bird flu, a viral disease of birds. Even the highly pathogenic strain H5N1 is overwhelmingly a disease that affects birds. Nearly all the small number of humans who have become infected were poultry workers who came into intimate contact with birds. Influenza viruses are constantly changing, and they might adapt over time to infect and spread among humans.