Halhiker is acclimated to the heat. The body is able to adapt to the heat but it takes time. Hikers coming into the desert and believing that because they are in shape its okay are risking a lot. I've seen Marines brought to 29 Palms from cooler bases drop out in the first week of arrival. Given 2 weeks to climatize and they are ready to start building their desert legs.
Its one of the reasons the Corps sends them to 29 Palms before going to the Middle East.

