Which of the following is a common sign of infection?

• A. Fever
• B. Headache
• C. Shortness of breath
• D. Weight gain

Answer: (A)

Fever is a common sign of infection. When pathogens invade, immune cells release pyrogens such as certain cytokines (IL-1, IL-6, TNF-alpha) and, in some cases, bacterial toxins. These pyrogens act on the hypothalamus, raising the body's set point for temperature by increasing prostaglandin E2. The result is an elevated body temperature. This systemic temperature rise helps the immune system work more effectively—accelerating immune cell activity and inhibiting some pathogens' growth. Because fever reflects this whole-body response to infection, it stands out as a typical and widely recognized sign.

Headache can occur with infections but isn’t as specific or reliably linked to infection alone. Shortness of breath may accompany certain infections, especially respiratory ones, but it points more to involvement of the respiratory or cardiovascular system. Weight gain is not a characteristic sign of infection and can indicate other issues like fluid retention or metabolic changes.