Trying to hurry in the heat and high humidity could lead to heat exhaustion, which can include mental confusion. Keep going, don't drink enough, go into heat stroke, and die.

Add an unknown medical condition or medication, and the situation could escalate even faster.

I doubt that it could have been snakebite, as someone mentioned. I've never heard of a case of snakebite without obvious symptoms, invisible even at a cursory autopsy.

Sue