**1. Introduction**

There are four types of influenza viruses, types A, B, C, and D, in the world. Influenza virus infection is zoonotic and sometimes restricted to specific species. Influenza A and B viruses mainly cause disease among humans, swine, and avian. Influenza C virus infects humans and swine but does not cause severe disease, and its infection to humans is rarely reported [1]. Influenza D viruses affect swine and cattle and are not known to cause sickness to humans [1]. Influenza B viruses infect humans and avian and cause epidemic in the limiting mild disease [2]. Influenza B virus is concerned about seasonal viral infection but not endemic or pandemic. Influenza A virus is different from other types of influenza viruses about the wide infectious ranges of host and severity of disease. Influenza A viruses are theoretically classified into thousands of different

antigenic subtypes by the combinations of the main viral antigens: hemagglutinin (HA) and neuraminidase (NA) [3]. Recently 18 different HA subtypes and 11 different NA subtypes have been identified [4, 5]. Influenza A viruses are classified from H1N1 to H18N11 and are called as avian influenza virus, swine influenza virus, or other types of animal influenza viruses depending on original infectious host. Humans, swine, and avian are infective host of influenza A virus, but infectious influenza A viral subtypes among intraspecies are restricted by the binding affinity of the HA protein to sialyloligosaccharides (oligosaccharides containing terminal silica acids linked to galactose) on host cell surface glycoprotein or glycolipids [6]. Influenza A viruses cause normally self-limited disease (asymptomatic or mild) and are shared with humans and many different animals. Most subtypes of influenza A viruses are primarily reserved in aquatic birds, are distinct from human influenza A viruses, and do not easily transmit to human due to host species barriers [7]. Zoonotic infection between humans and animals or interspecies of infection of influenza A viruses makes viral evolutions which can help to surmount species barriers [8]. Avian influenza viruses transmit from aquatic birds to poultry and change virus properties and result in change pathogenesis ranging from mild disease to severe disease with high death rates in host species. According to these phenomena, avian influenza viruses cause severe disease and high death rate in host, which are called as highly pathogenic avian influenza (HPAI) viruses, and avian influenza viruses result in mild disease or asymptomatic infection to host, which are called as low pathogenic avian influenza (LPAI) viruses. A virus is defined as HPAI or LPAI by its ability to cause disease and mortality in intravenously inoculated young chickens in the laboratory or by its possession of molecular characteristic features associated with HPAI viruses. Avian, swine, and other zoonotic influenza infections in humans may happen on the condition of the successful transmission with the sufficient contact between donor and recipient species, successful species jumps of influenza viruses, the influenza viral complex adaptations, and achieved sustained transmission in a new species [7]. Those may cause several ranges of disease in humans depending on viral factors and human factors [9]. HPAI virus infections have been monitored from avian to humans and caused the endemic disease of high fatality rate [10]. HPAI viruses may cause pandemic from endemic on the condition of no effective protection for wide shedding of HPAI viruses. Monitoring of disease severity of avian and human influenza A virus infection could prevent the severe damage to the society from the emerging infectious disease of influenza A viruses [9].
