Infections spread much faster in winter primarily because cold weather forces people indoors into enclosed, poorly ventilated spaces where viral aerosols easily accumulate and circulate. At the same time, the dry air common in heated indoor environments causes respiratory droplets to shrink into lightweight particles that stay suspended in the air longer, while simultaneously drying out nasal passages and reducing the protective mucus that normally traps invading pathogens. Furthermore, many viruses feature an outer protective membrane that hardens at cooler temperatures, allowing them to remain stable on surfaces and in the air longer, while shorter winter days lead to reduced sunlight exposure and lower vitamin D levels, which can subtly weaken our overall immune defenses.