During the pandemic last few years, the Infrared thermometer is widely used to measure the body temperature.

However, the fundamental question or the question in the first place is, why do we expect the radiation from our human body to be black body radiation?

As far as I know, the radiation of the earth is not a black-body radiation. See the link here

https://seos-project.eu/earthspectra/earthspectra-c02-p17.html