RE: Islam in South Korea.
According to Wikipedia ( sourced originally from a Korean article ):
https://en.wikipedia.org/wiki/Islam_in_Korea#Today
In 1962, the Malaysian government offered a grant of 33,000 USD for a mosque to be built in Seoul. However, the plan was derailed due to inflation. It was not until the 1970s, when South Korea’s economic ties with many Middle Eastern countries became prominent, that interest in Islam began to rise again. Some Koreans working in Saudi Arabia converted to Islam; when they completed their term of labour and returned to Korea, they bolstered the number of indigenous Muslims.
The Seoul Central Mosque was finally built in Seoul’s Itaewon neighborhood in 1976. Today there are also mosques in Busan, Anyang, Gwangju, Jeonju, Daegu and Kaesong. According to Lee Hee-Soo (Yi Hui-su), president of the Korea Islam Institute, there are about 40,000 listed Muslims in South Korea, and about 10,000 are estimated to be highly active practitioners.
But the infestation isn’t anywhere near the millions here in America or worse the problem Europe has.