Hello. Very often a change of environment can cause diarrhea,especially when this change is going from a colder country to a warmer one. The problem is cause by an imbalance of intestinal flora. The intestine is subjected to an increased amount of bacteria that may not be present in the home environment,hence the imbalance. Two extremely safe ways of preparing or countering this is by the consumption of probiotic substances. Many high tech probiotics should be stored at a certain temperature marked on the container,most often they are not stored at the correct temperature and consequently the product is compromised (dead) and is usually ineffective. The two products that i have relied on for the past 38 years of extensive travel are Natural yoghurt and root ginger. It is not nessecary to use both, one or the other will do,but root ginger is easier to carry in a backpack. If using ginger,take a piece the size of a cherry and either crush it between two stones or other hard surfaces,put it in a cup and pour boiling water on it (most cafes in Spain or Portugal will do this for you ) add honey for energy boost or just to sweeten. Natural yoghurt should contain some type of live culture,such as bifidus of some other lacto bacillus. Makeing this a part of daily routine a couple of weeks before the camino and during can be very helpful. Of course these are not medicnes ,so be vigil and if unwellness continues see a doctor.