I've already experienced this twice.

The first time was a year ago, on a hiking camp: on the first day I recklessly went out with one of the best groups there, and while I didn't lag behind, this was very exhausting. On the second day I had to stay in guesthouse due to moderate fever and vomiting. On the third and subsequent days, however, I was fine and could resume hiking, however I was choosing less advanced groups.

The second time was yesterday. I went out by foot to visit two neighboring towns, after spending a year with hardly any physical activity. This took nine hours of walking, still I succeeded. In the evening I again had moderate fever.

Why does this happen? Do other people also experience this, or is such a body reaction specific to me?

  • 2
    This smells a bit like dehydration is one of the underlying causes. Sep 12, 2016 at 12:23

3 Answers 3


Others experience this too (the internet is filled with similar stories). It does not look like you were really sick - this would have lasted longer than a day. However a weakened immune system is a concern after exhausting exercise.

It is difficult to tell you what happened to you, but we can still guess. For example people with high blood pressure can experience fever after too much unprepared exercise. You might want to get checked (and while you are at it, check other stuff too, your doc will be able to tell you what to look for).

The vomiting could be a side effect of feeling ill due to the fever, but it can't hurt to ask if you drank enough. Especially after long breaks of exercise. You tend to sweat more, and your body loses salts etc, electrolytes in general. This is no big deal for short workouts, but after long trips this may add up - and you will feel sick.

There are several possible reasons, but to be save you might want to get checked up by an expert, and "listen to your body signals" (thanks, mom!) when exercising.


Fatigue itself shouldn't cause fever, as it is really a symptom, not a cause, however being fatigued definitely leaves you less able to defend against illness, and an oncoming fever or illness makes you much more likely to be fatigued.

If you have spent a year without any exercise and then have a fairly intense exercise, your body can react in quite strange ways. It is a bit of a shock to your system, and cranking up your metabolism can feel a bit like a common fever.


Sunstroke or dehydration would also cross my mind. Think back to the environment, circumstances, equipment and weather you had then: did you miss a hat? was is a really hot day? did you drink enough water?

  • Due to the vomiting I would put my bet on a sunstroke too - I experienced the same.
    – Haini
    Sep 19, 2016 at 7:42

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