Different ethnic groups carry different genes. You know how you sometimes see headlines like "DNA of 2,000-year-old corpse in the Himalayas analysed; he turns out to be Greek"? So apparently some ethnicities have some genes that others don't; otherwise they couldn't know that.
Make a bioweapon that targets people with genes that your ethnic group doesn't have, and you've made a bioweapon that targets only people of other ethnic groups.
Different ethnic groups carry different genes. You know how you sometimes see headlines like "DNA of 2,000-year-old corpse in the Himalayas analysed; he turns out to be Greek"? So apparently some ethnicities have some genes that others don't; otherwise they couldn't know that.
Make a bioweapon that targets people with genes that your ethnic group doesn't have, and you've made a bioweapon that targets only people of other ethnic groups.