Cold and flu season doesn't wait for winter. The back-to-school wave hits while it's still warm outside, as dozens of kids enter close quarters with recirculated indoor air. The upside: The warm weather outside can help. Warm seasonal air carries more natural humidity than harsh winter air, working like a vaporizer to loosen mucus, soothe raw nasal passages and calm a dry cough. Brief time in late-season sunshine helps restore vitamin D, which supports the immune cells that fight off viruses. And sitting in warm air can relax tense muscles and ease the mild aches and chills of a low-grade fever more comfortably than staying shut inside an air-conditioned room.