Despite much effort in studies and preparedness from deadly viruses including HPAI, SARS and MERS outbreaks, the unexpected 2009 H1N1 strain and 2019 SARS-CoV-2 quickly spread worldwide and became pandemics in 2009 and 2020, respectively. Although H5 and H7 vaccines, either monovalent or bivalent forms, are used for protection of domestic birds [252], the ever-changing and highly reassorting HPAI H5 has continued to cause outbreaks in domestic birds and to be detected in dead wild birds [24]. HPAI H7N3 in poultry has been reported until now [24]. HPAI H5N6 human cases [253] and HPAI H7N9 human cases [254] were still reported in 2019. Several avian IAVs including H7N9, HPAI H5NX (X, NA subtypes) and H9N2 sporadically cross species to infect humans. MERS-CoV is still causing an endemic in dromedary camels and human infections [8] and SARS-like CoV is still circulating in animals, notably in palm civets and raccoon dogs [100]. Thus, it is very important to understand how a new pandemic arises.