Bird flu viruses usually do not infect humans, but can do so via a pig intermediate host.

Some flu strains may originate in birds, then undergo genetic recombination in a pig before propagation among humans.

The Hong Kong flu of 1968, for example, arose from recombination between duck H3N8 and human H2N2.

The recombinant H3N2 was a new strain never experienced in human populations before.