Complex sentence: Supporters of the conspiracy theory suggest that SARS caused the most serious harms in mainland China, Hong Kong, Taiwan and Singapore, countries where most Chinese reside, while the United States, Europe and Japan were not much affected.
Split into simpler sentences: Supporters of the conspiracy theory suggest that SARS caused the most serious harms in mainland China, Hong Kong, Taiwan and Singapore, countries where most Chinese reside. While the United States, Europe and Japan were not much affected.