Which vital sign is typically the first to change in response to illness?

Temperature is typically the first vital sign to change in response to illness due to the body's immune response. When an individual becomes ill, especially due to infections, the body may increase its temperature as a defense mechanism—a process known as fever. This elevation in temperature occurs because the hypothalamus in the brain raises the body's set point to create an environment less favorable for pathogens, thereby aiding the immune system in fighting off the illness.

Although changes in heart rate, blood pressure, and respiratory rate can occur in response to illness, they typically follow after the initial rise in temperature. For instance, an elevated body temperature might lead to an increased heart rate (to help distribute immune cells and nutrients) or changes in respiratory rate as the body attempts to manage metabolic demands and maintain homeostasis. However, the initial reaction, especially in response to a wide range of infections or inflammatory processes, usually manifests first as a change in temperature.