A single virus particle of influenza A is insufficient to initiate disease in humans. It is estimated that between 100 and 1,000 influenza A virus particles are necessary to cause the flu in an individual. A single droplet sneezed from an infected person is likely to contain sufficient virus particles to initiate the disease.

Explain why a single virus particle is insufficient. To do this, describe both the barrier and the cellular mechanisms of innate immunity that could play a role in stopping viral particles from initiating and then spreading flu within a vertebrate host.