Why Do We Get Fever When We Catch a Cold?

When you come down with a cold, one of the most common symptoms is fever. But what exactly causes your body temperature to rise during illness? The answer lies in your immune system's sophisticated response to infection. A fever isn't just a side effect—it's actually a strategic defense mechanism triggered by your body to fight off invading pathogens like viruses and bacteria.

The Science Behind Fever During a Cold

Fever occurs as a direct reaction to endogenous pyrogens—substances produced within the body, particularly cytokines, which are signaling proteins released by immune cells when they detect an infection. These cytokines travel through the bloodstream and reach the hypothalamus, the brain's command center for regulating body temperature.

How the Body's Thermostat Gets Reset

The hypothalamus normally maintains a stable "set point" for body temperature, typically around 98.6°F (37°C). However, when cytokines stimulate the production of prostaglandins in the hypothalamus, this set point gets elevated. As a result, the body interprets its current temperature as too low—even if it's normal—and kicks into action to raise it.

This process involves physiological changes such as shivering, reduced blood flow to the skin, and increased metabolic rate—all designed to generate more heat. That's why you often feel cold or experience chills at the onset of a fever, even though your internal temperature is rising.

Fever as a Sign of Immune Activation

Most colds are caused by viral infections, although bacterial infections can also trigger similar responses. When pathogens invade, the immune system responds rapidly by releasing inflammatory mediators. Fever enhances this response by creating an environment that's less favorable for microbial growth while simultaneously boosting the activity of immune cells like T-cells and macrophages.

In essence, fever is not an illness itself but rather a symptom of your body doing exactly what it's supposed to do: defending itself.

Normal Temperature Variations vs. Fever

It's important to note that body temperature naturally fluctuates throughout the day due to circadian rhythms, physical activity, and hormonal changes. In healthy individuals, these variations usually stay within a narrow range—about 1°F (0.5°C)—with lower temperatures in the morning and slightly higher ones in the evening.

In contrast, during a fever, the increase in temperature is more pronounced, often exceeding 1°F above the normal baseline. For example, a person might see their temperature climb to 100.4°F (38°C) or higher, signaling that the body has actively raised its internal thermostat to combat infection.

Fever during a cold is a well-orchestrated biological response involving immune signaling, brain regulation, and metabolic adjustments. Rather than viewing it as something to suppress immediately, understanding its purpose can help us appreciate how our bodies naturally strive to restore health.