In April 2005, a virulent strain of influenza hit a maximum-security forensic psychiatric hospital for men that’s midway between San Francisco and Los Angeles. John J. Cannell, a psychiatrist there, observed with increasing curiosity as one infected ward after another was quarantined to limit the outbreak. Although 10 percent of the facility’s 1,200 patients ultimately developed the flu’s fever and debilitating muscle aches, none did in the ward that he supervised. WINTER WOES. Cold-weather wear and the sun’s angle in the winter sky limit how much ultraviolet light reaches the skin. This can add up to a deficiency in production...