BOSTON, Feb. 25 - American scientists said Friday that they had discovered two new human viruses in Africa that belong to the same family, retroviruses, as the virus that causes AIDS. So far, the scientists said, the new viruses have not been linked to any disease, but they are being monitored out of concern that they or similar retroviruses might conceivably spawn another epidemic. The viruses, found in rural Cameroon among people who hunt monkeys and other primates, were probably transmitted from the animals through blood from bites and scratches received in hunting, butchering and keeping the primates as pets,...