I’ve seen enough disaster films to know how this pans out.
Virus mutates and gets out into the general population.
Smattering of cases becomes an avalanche.
Quarantine of a town or city.
Whole country is in self imposed quarantine.
Flights out, landing in other countries are checked and patients showing signs of infection are quarantined.
Infection spreads to other countries
And a world wide pandemic wiping out 75% of the population ensues.