I once heard that the human body, by virtue of its water content, absorbs some of the RF energy emitted by a typical bodypack transmitter. Since then I have found a definite pattern among customers complaining of inexplicable wireless problems which for no apparent reason seem confined to a particular user rather than a particular transmitter or receiver. Those users almost always turn out to be -- how shall I say -- corpulent, implying a more bountiful internal reservoir. UHF systems seem to suffer more than VHF systems, I suppose because of the shorter wavelength of UHF signals.
The human body is both a reflector and an absorber of RF energy. This is likely to be even more apparent at higher frequencies, e.g. 2.4 GHz.
Try this experiment. If you have a walkman FM radio, tune in a station and do a 360 degree spin. Note how the station fades as you turn your body.
So it is not surprising that placing a large human body between the transmitter's antenna and the receiver's antenna can cause degradation in RF performance.