As a 19 year + Army medic, I can tell you what I have seen the most in the Norhern and Southern Hemispheres:

North: dehydration, hands down the winner. Followed by contact dermatitis (poison ivy and such); relatively few cases of hypothermia.

South: dehydration, hands down the winner. Followed by fungal infections, gastro-intestinal disorders due to local food consumption, and sexually transmitted diseases.

For what it's worth, in the deserts north and south: dehydration and hypothermia seem to go hand in hand. Also, don't pick up the rocks, don't play with the snakes....and dang sure shake out your boots before you put them on!