Across the world, the smallest of creatures carries some of the heaviest consequences — mosquitoes transmit diseases like Zika and dengue that can reshape lives in an instant. Yet unlike many threats to human health, this one yields readily to informed action at the household level. Experts remind us that the boundary between safety and exposure is often as thin as a torn screen or a forgotten puddle of rainwater — and that closing those gaps is well within the reach of ordinary people.