The World Health Organization tried this week to dampen fears about mutations seen in the swine flu virus in several countries, noting that both mutations had been found in very few people. A change that created Tamiflu resistance has been found in about 75 people around the world, said Dr. Keiji Fukuda, chief flu adviser to the W.H.O.’s director general. Two clusters, in cancer units at Duke University Medical Center in North Carolina and a hospital in Wales, were both among patients whose immune systems had been severely suppressed by cancer treatment; some had had their bone marrow, which produces...