population of south korea: 52 million (2017)

population of united states: 326 million  (2017)

factor: 6.3

# of known cases in south korea: ~10000.
% of population: 10000/52000000 = 0.00019 = .02%. 

# of known cases in the united states: ~100,000

% of population: 100000/326000000 = 0.00030674846 = .03%