Fevers happen because the body is actively trying to fight off an infection or other threat, not just because something “goes wrong.” The brain deliberately raises the internal thermostat to make the body less comfortable for germs and to boost parts of the immune system that work better at higher temperatures.

What a fever actually is

  • A fever is usually defined as a body temperature of about 38°C (100.4°F) or higher.
  • The “thermostat” in the brain (the hypothalamus) resets to a higher temperature in response to signals released during infection or inflammation.

Why the body raises temperature

  • Many viruses and bacteria grow less efficiently at higher temperatures, so a fever can slow down how fast they multiply.
  • Higher temperature can enhance the activity of white blood cells and other immune processes, helping the body detect and clear invaders more effectively.

What triggers a fever

  • Common triggers include viral infections (like flu or COVID), bacterial infections (like strep throat or pneumonia), and sometimes vaccines as the immune system “practices” responding to a germ.
  • Fevers can also be caused by non‑infectious problems such as autoimmune diseases, certain cancers, some medications, and heat-related illness.

Why you feel chills and aches

  • When the brain decides the body should be hotter, it suddenly treats your current temperature as “too cold,” causing chills and shivering to generate heat.
  • Muscle aches, fatigue, and loss of appetite often accompany fever because inflammatory chemicals affect the brain and body systems, encouraging rest so energy can be redirected to immune defense.

Are fevers good or bad?

  • Mild to moderate fevers are often helpful and can be left alone if you are otherwise okay, because they support the immune response.
  • Very high fevers, fevers in very young babies, or fevers with worrying symptoms (trouble breathing, confusion, stiff neck, persistent chest pain, or dehydration) need medical attention promptly.

If you are ever unsure whether a specific fever is safe to watch at home or needs urgent care, checking with a healthcare professional is the safest option.