Hi. I would like to help you with your query. When we have fever or any infection , body immunity starts responding against foreign viruses or bacteria , which in response rises the body temperature. Once the temperature rises in our body , it causes water loss at cellular level with important electrolytes too like sodium and potassium. Sodium and potassium is one of the ingredients in contraction of the muscle. Excessive loss of potassium , makes the muscle fatigue as giving muscle pain as symptom. Also in medical term there is interleukins , which primarily produced by macrophages ( the cell scavenger). Interleukins cause pain , body ache and hypersensitivity of nerves . To different etiologies ,interleukins establish different responses. Hope I was helpful. Thank you.