HXNX is a way of indentfying large families of Orthomyxoviridae in particular alphainfluenza betainfluenza gammainfluenza and deltainfluenza, the four 'families' of bird/mammalian flus (often just called a,b,c,d) I forget off hand the exact proteins it refers to, but all of the viruses have one of four of them so H1N3 viruses tend to behave similar to each other but different from H1N2.

H1N1 is an alpha virus, that different strains have caused several major pandemics including the Swine Flu. It is an avian virus usually, but some strains are endemic in humans and it is often the flu-a vaccine for a year.

1918 flu is an outlier as was the '83? '82? Russian pandemic novel. The 2008 was slightly more lethal than normal but not more contagious.