A Chinese law banning the buying and selling of blood took effect. The law reflects a growing uneasiness in China about the heavy reliance on paid blood donors. This reliance is contributing to the rapid spread of AIDS. Medical centers all over the country are bracing for problems because of the law. To meet rising medical needs, hospitals use a system in which poor people sell their blood, sometimes to middlemen who falsely declare that the blood products have been screened for disease.