https://en.wikipedia.org/wiki/Heat
People seem to emit infrared light, but that is not the light that people usually see through an infrared camera.
https://www.livescience.com/7799-strange-humans-glow-visible...
This is showing infrared and even visible light radiation from a person's body in extremely small amounts.
An infrared camera just sees infrared light period, the vast majority of which is what is reflecting off of someone from other sources.
https://en.wikipedia.org/wiki/Heat