Influenza A (H1N1) virus (Fig. 2.1) can be classified into orthomyxoviridae, the genus of influenza viruses A, and is linear single minus-strand RNA with varying shapes. In general, it is an envelope virus in sphere shape, with a diameter of about 80–120 nm. On the surface of the envelop, there are many protuberant glycoproteins in radial arrangement, namely spikes and fibers which are column liked hemagglutinin (HA), mushroom shaped neuraminidase (NA) and membranous matrix protein 2 (M2). Inside the viral particles is helically symmetrical nucleocapsid, with a diameter of 10 nm. The ring shaped structures of its both ends are in the viral envelope, containing eight different sized independent segments and the genome size of about 1,316 kb. Influenza A (H1N1) virus is susceptible to inheritably mutation due to its multiple serotypes, whose mechanism involves molecular antigen drift and shift.

The Influenza A (H1N1) virus is an enveloped virus, being sensitive to organic solvents such as diethyl ether, chloroform and acetone. Its infection power can be destroyed in 200 ml/L diethyl ether at 4 °C overnight. As the virus is also sensitive to oxidant, chloride, heavy metals, ethanol and formaldehyde, it can be inactivated in 750 ml/L ethanol for 3 min, in 1 ml/L iodine tincture for 5 min, or in 1 ml/L formaldehyde for 30 min. In addition, the virus is sensitive to heat, being inactivated at 56 °C for 30 min; and to ultraviolet rays, but the virus being inactivated by ultraviolet may have multiple reactivation.

Figure 2.1 is an electronic microscopic photo of round Influenza A (H1N1) virus released by the CDC of the US, showing the viral particles.