Why does a fever start with chills?

The question

Her temperature is going up, yet she's shivering and asking for a blanket: is her body getting it wrong, and is the fever the enemy?

Understand

When a fever starts, germs set off chemical signals that turn up the brain's thermostat (the hypothalamus). The body is suddenly cooler than its new setting, so it narrows the blood vessels in the skin and shivers to make heat: that's why the chills come first, even while the temperature is climbing. The extra heat can slow some germs and helps parts of the immune system work, so a fever is part of the body's defence. When the thermostat turns back down, the body is warmer than the setting, so it flushes and sweats. A fever is still a signal of infection, so look at the whole person, not just the number. Normal body temperature is usually about 36.5 to 37 °C, and a fever is 38 °C or higher. Older adults may not get a fever even with a serious infection. Mrs Smith, Alex and all numbers are fictional examples, and the drawings are simplified teaching schematics. Charts and rules vary, so follow your local policy.

Get help urgently for a fever with confusion, a seizure, trouble breathing, a stiff neck or a rash that doesn't fade, for a baby under 3 months with a fever, or for anyone with a fever or chills who is getting worse quickly (these can be signs of sepsis).

1. Germs set off signals (pyrogens, then prostaglandins) that raise the set point of the hypothalamus: the body's thermostat.
2. Now cooler than its new setting, the body keeps and makes heat: skin vessels narrow, muscles shiver. That's the chills.
3. The extra heat can slow some germs and help some immune cells.
4. When the setting comes back down, the body is warmer than it, so it flushes and sweats.

Key takeaways

• Normal is usually about 36.5–37 °C; a fever is 38 °C or higher.
• Chills while the temperature rises are the body heating up to a raised setting, not a mistake.
• A fever can signal infection, and fever or a low temperature with chills can be a sign of sepsis: check the other obs, look at the whole person, report it.
• Older adults may not get a fever, even with a serious infection.

In practice

• Rising temperature and shivering? Check the other obs and look at the whole person.
• Fever with chills can be a sign of sepsis: tell your supervising nurse.
• Older adults may not get a fever, even with a serious infection.

Check yourself

38.4 °C and rising, and asking for another blanket. Is the body getting it wrong?

Words to know

Fever

38 °C or more (normal: usually about 36.5–37 °C)

Hypothalamus

the part of the brain that sets body temperature: the thermostat (hypo- (below) + thalamus (a brain area))

Pyrogen

something that causes fever (pyro- (fire, heat) + -gen (producing))

Transcript

Sources

1. StatPearls. Physiology, Fever (NCBI Bookshelf): exogenous pyrogens stimulate IL-1, IL-6, TNF and IFN to alter the hypothalamic set point; fever is mediated by PGE2; growth inhibition of specific microbial agents; increased motility and activity of white blood cells; a 1 °C rise needs a 10-12.5% higher metabolic rate (more oxygen demand, heart rate, breathing rate); fever may not be seen in older adults; in hyperthermia the set point is unaltered (read 12 October 2026)
2. StatPearls. Physiology, Temperature Regulation (NCBI Bookshelf): pyrogens act on the hypothalamus and release prostaglandins, which increase the set point; vasoconstriction of skin arterioles and shivering keep and make heat; benefits of fever include inhibiting bacterial growth and increasing the efficiency of immune cells; infection-induced fevers are often absent in older adults (read 12 October 2026)
3. MSD Manual Professional. Fever: fever results when something raises the hypothalamic set point, triggering vasoconstriction and sometimes shivering until the blood reaches the new set point; a falling set point initiates heat loss through sweating and vasodilation; fever inhibits the growth of some microorganisms; experimental evidence, but not clinical studies, suggests fever enhances host defences (read 12 October 2026)
4. healthdirect. Fever: normal body temperature usually 36.5-37 °C; a fever is 38 °C or higher; a fever is a sign your body is working to fight the infection; go to emergency with fever plus seizures, confusion or difficulty breathing (read 12 October 2026)
5. healthdirect. Sepsis: signs include fever or low temperature and chills; if you are quickly getting worse, call triple zero (000) (read 12 October 2026)
6. Healthify NZ. Fever in adults: a fever is your body's way of fighting infection; you can feel very cold and shivery even when your temperature is high; older adults may not get a fever even with a serious infection; call 111 for an emergency (read 12 October 2026)
7. Royal Children's Hospital Melbourne. Fever in children (Kids Health Info): fevers are a sign the immune system is working; no need to treat a fever if the child seems comfortable and is drinking; lowering it won't make the infection go away faster; a baby under 3 months with a fever above 38 °C should see a doctor as soon as possible (read 12 October 2026)

Checked against these sources at the time of publishing. Sources change, so check the date above and your local policy. Spotted an error? Tell us and we'll correct it.