[XorNot]

No this is all statistics again: probability that a viral particles survives in the body, probability that it successfully enters the cell (this is all chemical kinetics so it's not 100%), probability the cell survives long enough to replicate the virus, probability of an immune response picking up the lyzed cell immediately and then repeat for the daughter copies.

An analogous process would be human fertilization: it technically only takes 1 sperm to fertilize an egg, but it's millions in order to make the probability of it happening meaningfully high enough.

Of course some viruses are stupidly good at this: it's estimated 5 norovirus particles will trigger a full blown infection.