Avian Influenza
Pandemic influenza viruses originate from avian sources - influenza A, which first evolved in birds, can occasionally infect other species including pigs, cattle, and humans. The primary mechanism for bird flu transmission to humans involves 'reassortment' with human influenza viruses. This process creates a hybrid virus that maintains human-adapted characteristics while incorporating novel bird virus antigens that can bypass human immune defenses. Due to their susceptibility to both human and avian viruses, pigs often serve as mixing vessels for this reassortment process. While humans can directly contract animal viruses, the greatest pandemic risk occurs when a person is simultaneously infected with both bird and human flu strains. Each viral replication event in such cases provides another opportunity for reassortment to produce a virus that combines human adaptability with bird-derived antigenic properties.
