Heatstroke can follow heat exhaustion if left untreated. Heat exhaustion can cause a high fever, irregular heartbeat, dizziness, nausea, fainting, thirst, etc. The body is trying to cool itself down. Heatstroke is when the body's cooling system has shut down and the condition can become life threatening with some of the same, and more extreme, symptoms including vomiting, headache, fatigue, shortness of breathe, etc.
