Answer:
Because the H7N9 avian virus acquired pathogenic mutations in order to infect mammalian hosts
Step-by-step explanation:
H7N9 is a subtype of influenza avian viruses that has recently been reported to infect humans. Fortunately, H7N9 has also been reported to have a low pathogenic level; however, mutations might change this level to become more infectious for humans. Consequently, mutations in a given H7N9 strain might enable its adaptation and higher affinity to human-type receptors, being more aggressive to human cells, especially in lung cells, since this virus is characterized by rapid progression and respiratory failure.