[tzs]

The infectious dose for most viruses is higher than that. See [1] and [2].

[1] https://www.erinbromage.com/post/the-risks-know-them-avoid-t...

[2] https://en.wikipedia.org/wiki/Minimal_infective_dose

[ponker]

It's a probabilistic process. 1 virion of any virus can infect a host, but for some viruses that probability will be vanishingly small while in others it will just be regular small.