Have you noticed that the so-called Holy water doesn't get stale? Ever wondered why?
I heard that the process is explained by simple physics and chemistry, the bottom line being: the cross used for this, is made of silver and when it interacts with water, the water molecules change their structure and become purified and more "positively" charged. And of course, the belief plays an essential role, but it;s not the only factor in my view.
Remember that human body is mainly water...