禽流感
Avian influenza A H7 viruses are a group of influenza viruses that normally circulate among birds. H7 influenza infections in humans are uncommon, but have been confirmed worldwide in people who have direct contact with infected birds. Most infections have been mild involving only conjunctivitis and mild upper respiratory symptoms. The avian influenza A(H7N9) virus is a subgroup among this larger group of H7 viruses. Although some H7 viruses (e.g. H7N2, H7N3 and H7N7) have occasionally been found to infect humans, H7N9 has previously been isolated only in birds, with outbreaks reported in the Netherlands, Japan, and the United States. Until the 2013 outbreak in China, no human infections with H7N9 viruses had ever been reported.