Rehydration can still be accomplished when a person cannot take anything by mouth. Normally he would be given an IV. In a SHTF situation, this may not be possible. Rather one can fill an enema bag with normal saline (very dilute salt water-recipe on the internet-need not be sterile) attach a rubber urinary catheter to the enema tip, Using vaseline jelly feed the rubber catheter into the rectum carefully as not to pierce the bowel and let the tepid salt water very slowly drip in. It should be able to be absorbed because it is going in very slowly and a little higher up rather than being forcefully expelled as would happen if an irritating solution was used to treat constipation. This should only be done of course when the benefits outweigh the risk. - Sharon G.