Visas: With an onward ticket visitors from almost anywhere - except countries not recognized by South Korea (Cuba, Laos & Cambodia) - can stay in the country for 30 days without a visa. If you're from Western Europe or Canada, you can get up to 90 days visa-free. Everyone else has to extend after their first 30 days. Extensions last for around 90 days, and if you know you're going to need one it's worth getting it before you come.
Health risks: no particular risks, but you should consider vaccinations for hepatitis, typhoid, polio, tetanus & diphtheria