LONDON — A genetic variant commonly found in Chinese people may help explain why some got seriously ill with swine flu, a discovery that scientists say could help pinpoint why flu viruses hit some populations particularly hard and change how they are treated.

Less than 1 percent of Caucasians are thought to have the gene alteration, which has previously been linked to severe influenza. Yet about 25 percent of Chinese people have the gene variant, which is also common in Japanese and Korean people.