IR sensitivity depends on a number of things- wavelength, range, humidity (attenuation), bandwidth, collection area, and the type of detector. A cryogenically cooler IR detector can see differences in temperature of millidegrees.
An example to put this into something the average person can get their head around. When you walk barefoot down a carpet, your feet contact the carpet for a second or two each step, transferring your body heat to the carpet, changing the carpet temps where you make contact by some very small amount. This temperature change is a small fraction of a degree C, sometimes only a few millidegrees, or a few 10's of millidegrees. Now that may not mean much to you, unless you already have a concept of what is happening.
A picture helps get the concept across. In the following image my youngest daughter walked on the carpet at a normal walking pace about 30 seconds before this image was captured. Her footprints, visible in this image, are the resultant temperature change to the carpet from the momentary contact of each step. The sensor used was a FLIR Indigo Systems, Merlin Mid IR (3-5 u), cryogenically cooled InGaAs.
Another example, same imager. In this image I reached out and put my thumb on the barrel of this camera lens and counted to 2, so roughly a 2 second contact with my skin. The bright spot in the image is the temperature change caused by that contact.
Think about how little, minute, the temperature change was in each of those events, and yet it is still clearly visible.
T!