This is a time of year when we physicians have to remind ourselves to pay attention and readjust our collective focus. Human illnesses are often quite seasonal, whether that means allergies in the spring and autumn, flu and pneumonia in the winter months, or insect bites in the summer. However, a unique danger arises when the thermostat drops. That danger is carbon monoxide, a dangerous byproduct of the combustion of burning carbon-based fuel. While many people have all-electric homes, plenty of others will be heating with natural gas, propane, charcoal, kerosene or wood stoves. All of these make homes cozy on cold days and colder nights. Propane or gasoline fuel generators will be also used more in the cold months and can also serve as another source of carbon monoxide.