Almost every disease leads to fatigue, partly due to that the immune system commences its work by sending out neurotransmitters that cause fatigue. The reason is probably that a body which is fighting an infection shouldn't be wasting vital resources on unnecessary and energy-consuming activity.

The fatigue may have a cause

Fatigue is one of the main components in someone's perception of being sick. There are also of course a number of illnesses that have fatigue as the primary symptom. These include chronic fatigue and burnout, but also depression, MS and cancer also have strong elements of fatigue. Many changes in physical status also contribute, such as vitamin deficiency, iron deficiency, low levels of thyroid hormones or stress hormones. Fatigue is also caused by sleep deprivation, physical activity, lack of physical activity/exercise, monotony, and much more. Today there are no clear biological markers of fatigue, so you must put your trust entirely into subjective self-assessments.

Counteracting fatigue

The main countermeasures against fatigue are resting or changing to a completely different activity. Burnout (fatigue syndrome) is an extreme form in which it is no longer enough to rest or change activity to recover. A precise definition of fatigue doesn't exist today, but most attempts to define it concern the inability to continue an activity that has been going on for a (too) long period of time. The adjectives that are used include, in addition to "fatigued", physically tired, exhausted, and similar expressions.