We need to know the immunity of a community. One person infects 2-3 and they infect 2-3 and they infect 2-3 and they infect 2-3! SEE?

Until we are told 40% of the people have been infected by which time the number of people that can be infected has been reduced and so at some point 1 person will infect 2  and they will infect 2 and they will infect 2 and then one will infect one will infect one will infect one. Until it runs its course. How long will that take no one knows.