3 issues resulted in this:

1 - Food production/preparation. Korean mussels are grown in collected human shit. They are banned from import into the US because they are frequently found to be contaminated with norovirus (which can be destroyed with heat, but none the less, gross). Korea does not have a modern system of food safety training and inspection. It barely has an archaic one.

2 - There is very little education in Korea on personal hygiene and infection control. Practical science education is almost nonexistent, and people are largely informed by their social networks which relay bad and incorrect information along with superstition and viral rumours.

3 - The culture of coming to work while sick, or continuing to take care of one's normal responsibilities while sick. That includes caring for children, the elderly, and other relatives. In other words, mum is making supper or doing the grocery shopping while vomiting and having diarrhea.