What would happen if you had a normal, non-vaccinated human and a person with zero immunity from vaccines

Let's say the non-vaccinated human has an illness. The non-vaccinated person gets the illness and passes it on to the other person with no immunity. That person gets sick and also passes the illness on to another person and so on. So if there are 2 people, the illness will spread fast. But what if there is 1,000,000,000 people? The illness would spread fast.