Yes, more calories are being burned to keep a normal body temperature in a cold environment. Heat is constantly transferred from your skin to the environment, as long as it’s colder than your body’s temperature outside. This heat needs to be replaced somehow, otherwise you would get hypothermia. There are specialized cells called brown fat cells which help generate this heat. Whenever your body makes energy, there is always a small amount that is released as heat. In brown fat cells, their energy-making process is “leaky” so most of the energy is released as heat, rather than chemical energy that can be used later. Additionally, you can activate a large number of muscles to cause shivering which increases body temperature by expending a large amount of energy, some of which turns into heat. Shivering is basically like exercising in place that raises your body temperature when you’re cold. When you get a fever (usually because of an infection) your body does need to burn a lot of energy. This is a result of conservation of energy; if your body is hotter, it has more energy and that energy needs to come from somewhere. When you have a fever, you often feel cold. This is because your body has changed what normal is from around 98 degrees F to greater than 100 degrees F. As long as you’re below this normal point, you’re going to feel cold and as a result, going to shiver. It’s thought that the high temperature of a fever interferes with virus reproduction as well as increasing the efficiency of the human immune cells.

In short, yes. Your basal metabolic rate, which is the daily amount of calories you burn by just existing, increases a tiny bit when it starts to get colder. That’s because your body needs to try a little harder to stay warm. On top of that, things like shivering increase calorie expenditure considerably more. For the most part, though, the extra calories burned by the cold weather are minuscule in comparison to the amount burned by things like activity or exercising.

Infection does raise body temperature, usually causing a fever. A fever is the body’s defense against the infection. When your body temperature increases considerably, so does our metabolism. As each degree of temperature rises, energy demand from the body increases, which is why intake of food is important when sick to fuel and help heal the body.

Yes, our bodies burn more calories in cold weather to keep warm, and yes, our bodies burn more calories to increase our temperature when we have fevers. This is, in fact, why you feel tired when you have a fever: you are burning all of your energy to give yourself that extra boost in temperature.