Understanding of Host Switch and Host Adaptation to the Avian Influenza Viruses

During recent decades, several times of frequent appearance of the "newly emerging or re-emerging" pathogens are posing a great threat to the human populations as well as many other animals. It is well known that most risky groups are viral pathogens. Among them, taking a pose a striking threat to the human is belong to the influenza A viruses (IAVs). Influenza or flu is a wide spread zoonotic disease caused by influenza A virus, which has their ultimate origin from avian reservoirs and may stably adapt, either directly or after several passages thru another mammalian species of host, to human populations. Novel human-adapted IAVs have emerged to cause pandemics several times in the last 100 years. Typical avian influenza A viruses are restricted from replicating efficiently and causing disease in humans. Mechanisms by which viruses evolve in one host, cause host switch, and adapt to a new host species from wild aquatic waterfowl to mammalian species including human, remain to be elucidated. Here, some insights into the mechanisms of IAV"s host switch and their adaptation and viral virulence factors associated with a novel virus in human are briefly reviewed.