What is the most common cause of pyrexia (fever)?

The most common cause of pyrexia, or fever, is infection. When the body detects an infection, whether it be viral, bacterial, or fungal, the immune system responds by releasing substances called pyrogens, which act on the hypothalamus to raise the body's temperature. This increase in temperature can create an environment that is less favorable for pathogens and enhances the body’s immune response, helping to combat the infection more effectively.

Infections are the primary reason healthcare professionals consider fever as a clinical sign; it is often one of the first indicators that the body is fighting off an illness. While dehydration, allergic reactions, and medication side effects can indeed lead to elevated temperatures, they are significantly less common in comparison to infections. Thus, recognizing that infection is the foremost cause of fever is essential for clinicians when diagnosing and treating patients.