Three new international studies detailing how patients became gravely ill with swine flu reinforce concerns that U.S. intensive-care units could be severely stressed as the second wave of the disease builds through the fall and winter. The studies of patients in Canada, Mexico, Australia, and New Zealand were reported a week after the vaccine against the H1N1 flu began being distributed in the U.S. The Centers for Disease Control and Prevention says the disease has become widespread in 37 states.