Lots of infrared rays are given off by very hot objects. When these infrared rays are absorbed by something, its temperature rises. Our bodies radiate a range of infrared frequencies. The frequency of the radiation depends on the temperature of the part of the body concerned. This helps infrared cameras to 'see in the dark'.
Physiotherapists use infrared radiation to warm damaged muscles and so speed up healing.
Infrared light can also be sent down optical fibres for cable television and phone links.