Take the example of AIDS. Twenty years ago researchers in the US realized that blood transfusions were one factor in the transmission of the virus. Having found higher ratios of infection among homosexuals and Haitians, they stopped the medical use of blood from these groups, with the result that there was a 90 % drop in infection by means of blood transfusion.
以愛滋病為例，二十年前稱為「後天免疫不全症候群」，研究人員發現這種病毒在海地人及同性戀者身上比例相當高，懷疑傳染方式與輸血有關，於是下令海地人及同性戀者不能輸血，成功地阻止九成的輸血感染愛滋病。