The only reason the surplus has gotten so huge is because imports are falling much faster than exports.
This suggests that the South Korean economy is getting really soft, especially considering that the won has been getting stronger. (A strengthening currency usually has the opposite effect on trade since it makes imports relatively cheaper.)
The most immediate sector of South Korea’s economy that is likely to be hit by the MERS outbreak is tourism. But as Hong Kong’s 2003 SARS outbreak showed (pdf, pg. 2), a prolonged epidemic of an infectious disease can also cut domestic consumption and exports, and ultimately impact the local property market.