Quick fever screening at a mass scale is proven effective during a pandemic to single out the ones with suspected symptoms of infectious diseases. However, achieving affordable and real-time fever alert at an individual level, although more preferable, remains elusive. Herein, we report an inexpensive and highly sensitive fever detector, which possesses a sharp color transition temperature window tailor-tuned for fever screening. The sensing component of the detector is rationally designed thermoresponsive agarose@poly(N-isopropylacrylamide)-co-acrylamide hydrogel. The hydrogel turns from transparent to opaque white when its temperature is higher than its cloud point. As a proof of concept of its practical applicability, a wearable fever monitoring device was fabricated in the form of a wristband. When the wrist temperature is higher than the threshold of a human fever, the device shows a remarkable color change, alerting an elevated body temperature. The wearable detector provides a promising strategy for real-time fever alert monitoring and is capable of making contributions to inhibit the spread of infectious diseases during a pandemic.