How did H1N1 Influenza 09 infect humans?
Different types of influenza are commonly circulating in our environment, including types that can cause illness in humans, birds and pigs. Sometimes, types of influenza can pass between humans and animals through close contact, such as in pig farms and livestock exhibits. When pig or bird influenza viruses infect a human, there is a risk that the animal influenza can mutate (change its genes) and then spread directly between humans. The reason for the development of human swine influenza in Mexico is not known.
