LONDON Several teams of scientists around the world have, for some time, been studying the possibility that a genetic mutation perpetuated by the organism responsible for bubonic plague, or the Black Death, in the Middle Ages - Yersinia pestis - might give people now carrying the mutation increased resistance to the Human Immunodeficiency Virus (HIV) compared to non-carriers. New research has thrown doubt on the micro-organism that was thought to have caused the Black Death, but the link to HIV resistance seems to remain.