This isn't true. Even a robust immune system cannot prevent disease at large enough doses of virus because the immune system takes time to execute at any level

This isn't true. Even a robust immune system cannot prevent disease at large enough doses of virus because the immune system takes time to execute at any level of robustness. Similarly, a small enough dose will not necessarily cause disease, because the immune system can recognize the virus and eliminate it before it can replicate significantly. This is actually a time-based race: the speed of which the immune system can react vs the speed in which the virus can replicate in the body.