What I mean as external heat source, not just a cause of a fever. An infection is not actually hot itself, your body temp is rising in response to it. In the case of a heatstroke, there's literally heat being added to your system, not just your body responding to stimuli.

It's like with hypothermia, your body isn't cooling down on its own, the freezing air/water is doing it.

While good and interesting info, I feel like that doesn't qualify as on its own. With a heatstroke aren't their external factors raising the body temperature?