The virus has probably disappeared many times in many countries.

What's stopping [them] from simply putting a drop of c19 in a batch of heroin and distributing it in, say the USA?

What's stopping [them] from mailing someone a bag of seeds that might have c19 on the bag/envelope?

What's stopping [them] from flying a diplomat infected with c19 in and have them spit on everything for the first 12 hours?

Nobody would even know.

How the hell do you stop this from going on forever?