The tube filled with mercury inside a mercury thermometer reacts to the body temperature. It expands and reaches the temperature same as your body’s, which you can read on the stick. In contrast, digital thermometers react to the infrared rays that every human body produces. The infrared measurement could be different in contactless products like forehead thermometers. The circuitry inside the product consists of a thermistor, which is responsible for measuring the infrared rays and converting the reading into digital units. The sensors can be other than a thermistor. This is just one example. After reading the digital display, you can then take preventive measures if required. Other than technical disparities, there is no other difference in digital thermometers as such.