What is the difference between seasonal influenza, avian influenza, swine influenza and an influenza pandemic?

Influenza viruses are commonly circulating in the human and animal environment. Different strains can cause illness in humans, bird and pigs.Seasonal influenza is caused by influenza viruses that are adapted to spread in humans (human influenza). Humans have some natural immunity to the strains that are in common circulation, and this immunity can be boosted by immunisation with a seasonal influenza vaccine.Avian influenza is caused by influenza viruses adapted for infection in birds. Similarly, swine influenza is caused by influenza viruses adapted for infection in pigs.These illnesses all elicit the same respiratory symptoms in their hosts. Sometimes, humans and animals can pass strains of influenza back and forth to one another, such as when humans become ill with avian or swine influenza, usually from direct contact with animals who are ill.Mixing of human and animal influenza viruses can lead to the development of changed viruses with the ability to cause infection and spread in the human population. There may be little or no immunity in the human population to these new viruses.An influenza pandemic is defined as a new or novel influenza virus that spreads easily between humans. When new influenza viruses are introduced into the environment, humans don’t have any natural immunity to protect against them. Therefore, there is a risk that that new influenza viruses could develop into a pandemic if the virus passes easily from human to human.