Abstract
Influenza A viruses infect many animal species and have great zoonotic potential. The epidemiology of swine influenza worldwide is of particular importance because the pig can act as a "mixing vessel" where avian and human influenza viruses can undergo genetic reassortment, creating new viruses. Zoonotic transmission allows new strains of influenza A viruses to be introduced into the human population, potentially causing the next influenza pandemic. The dynamic nature of swine influenza viruses poses challenges to both the swine industry and public health as a source of zoonotic infection. Human infections with swine influenza viruses are regularly reported. Many of these zoonotic events have occurred through close contact between humans and pigs, particularly at agricultural fairs, which have become a source of emerging of swine-origin influenza A viruses. These unique pig-human interfaces have caused the majority of human infections with variant influenza A viruses. In this review, we examine zoonotic spread of influenza A viruses of swine origin, transmission of influenza viruses from pigs to humans worldwide, and the reasons for the emergence of zoonoses.