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Reference topic 20: The Fever and the Old Enemies

Should you let a fever run its course? What fever is for

ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.75 sources6 graded recommendationsTopic hub: Gut and immunity

Published Updated

In 1974 Matthew Kluger and two colleagues reported what sounded like a contradiction: a fever in a cold-blooded animal.1 Their subject was the desert iguana, a lizard of the American Southwest.2 A lizard makes little heat of its own.

The next year Kluger's team infected desert iguanas with bacteria and kept them at fixed temperatures. Most lizards kept at fever heat survived. Most kept cool died.3

Mismatch card

The system

Fever, sickness behavior and the defenses against infection.

What it expects

Infection as the commonest threat, care from others while sick, and bands too small for crowd diseases.

What it gets

Crowd and herd diseases, fever treated as the enemy, work while sick, antibiotics overused and vaccines refused.

What happens in the gap

Fever phobia and contagious presenteeism; returning measles; resistant bacteria; old genetic trade-offs meeting new viruses.

The handrail

Treat comfort and danger signs, not the number; rest at home; use antibiotics only when needed; vaccinate (●●●)

Strength of evidence

●●○ Fever aids survival in animals, but treating it has not changed survival in human trials, and selection claims are contested; vaccines and stewardship rest on strong data.

What this topic covers

  • ·A Thermostat, Reset
  • ·The Program Called Feeling Ill
  • ·The Number Is Not the Illness
  • ·Crowds and Herds
  • ·Scars in the Genome
  • ·The Fever That Broke
  • ·Forgetting Measles

The practical version is in the book: Chapter 6, The Gut That Expects Dirt.

The handrail

Each recommendation carries the strength of the evidence behind it. How the grades work.

  1. Treat the person, not the thermometer

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Use paracetamol (acetaminophen) or ibuprofen when fever makes you or your child miserable, not to hit a number. For children, give one drug at a time, not both together. Do not sponge them, strip them down or bundle them up, and do not use fever reducers to try to prevent seizures.67 Adults should take no more than 4,000 mg of acetaminophen a day from all sources, cold remedies included.68 No aspirin under 16 unless a doctor prescribes it.69 In dengue areas, use paracetamol, not ibuprofen or aspirin, until dengue is ruled out.70

  2. Rest, drink and stay home while feverish

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Rest and fluids are standard self-care,71 and staying home protects others. No trial shows that rest shortens a fever.

  3. Know the red flags, and call a clinician when in doubt

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Get prompt medical help for a baby under 3 months with 38°C (100.4°F) or more, or 3 to 6 months with 39°C (102.2°F) or more. Do the same for a fit, a rash that does not fade when pressed, fever lasting five days or more, or a child who seems to be getting worse.67 Confusion, breathlessness, a racing heart or weak pulse, or clammy skin can mean sepsis (blood poisoning) and need prompt help too.72 So do fever in pregnancy73 and fever after travel to a malaria area, even months later.74

  4. Skip fever reducers "just in case" at vaccination

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    They blunt the antibody response.27 Treat a distressed child afterwards as for any fever.

  5. Use antibiotics well

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    Do not ask for them for colds or flu, and never take leftover or borrowed pills. Experts are rethinking the rule to always finish the course, so ask your prescriber how long you need.75 Never refuse or stop an antibiotic a clinician says you need.

  6. Vaccinate on schedule

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Follow your national schedule for children and adults; if you are pregnant or have a weakened immune system, ask about live vaccines.

The skeptic's corner

The case against this topic, from the research itself.

"Let fevers run" is only half right. The animal case is strong but has a ceiling. In Kluger's rabbits, survival rose with fever only up to a point. In the same study, rabbits given fever reducers died less often, perhaps through some other effect of the drugs.59 In intensive care, infected patients whose temperature peaked around 39°C had the lowest adjusted death rates,60 yet lowering fever in trials changed nothing. In one trial in septic shock, where infection has sent blood pressure crashing, cooling cut deaths at two weeks from about a third to about a fifth.61 A plausible reading: a patient who can mount a fever has reserves, and in the sickest, fever's energy cost can outweigh its benefit.

Fever also frightens for real reasons. Febrile seizures, fits set off by fever, affect 2-4% of children, and fever reducers do not prevent them.62 Nearly one in five children who reached an emergency department at 41.1°C (106°F) or more had a serious bacterial infection.63 The danger lay in the infection behind the number. Heat stroke, when the body cannot shed heat fast enough, is not fever: it is an emergency.

The genomic scars need the same care. Six geneticists have since argued that the Black Death evidence does not hold up. The immune signal vanished under a stricter test, and the ERAP2 variant is as common in Britons today as in England long before the plague.64 The original team replied with new data and still ranks ERAP2 the strongest candidate.65 An earlier story, that plague favored the HIV-resistance mutation CCR5-delta32, has already collapsed.66 A few hundred ancient genomes are a thin basis for detecting selection; longer series from single places would settle more.

Terms this topic defines

Related topics

Sources cited above

  1. 1.Vaughn, L.K., Bernheim, H.A., and Kluger, M.J. (1974). "Fever in the lizard Dipsosaurus dorsalis." Nature, 252(5483), 473-474. https://doi.org/10.1038/252473a0
  2. 2.Bernheim, H.A., and Kluger, M.J. (1976). "Fever and antipyresis in the lizard Dipsosaurus dorsalis." American Journal of Physiology, 231(1), 198-203. https://doi.org/10.1152/ajplegacy.1976.231.1.198. Temperatures returned to normal on days 6 and 7. (The University of Michigan chamber ran at 12°C at night and 30 to 55°C by day depending on position; 13 infected lizards moved to warmer spots and ran a daytime fever averaging 2.3°C for five days.)
  3. 3.Kluger, M.J., Ringler, D.H., and Anver, M.R. (1975). "Fever and survival." Science, 188(4184), 166-168. https://doi.org/10.1126/science.188.4184.166. Low temperatures were 34°C or 36°C and high temperatures 40°C or 42°C. The survival percentages that circulate in secondary accounts disagree with one another and are not given here. (The bacterium was Aeromonas hydrophila; lizards were held from a cool 34°C to a feverish 42°C. The authors concluded: "An elevation in temperature following experimental bacterial infection results in a significant increase in host survival.")
  4. 59.Kluger, M.J., and Vaughn, L.K. (1978). "Fever and survival in rabbits infected with Pasteurella multocida." Journal of Physiology, 282, 243-251. https://doi.org/10.1113/jphysiol.1978.sp012460. The authors suggested the drugs may have helped through some effect other than lowering temperature. (Survival rose with fever up to about 2.25°C and fell above it.)
  5. 60.Young, P.J., Saxena, M., Beasley, R., Bellomo, R., et al. (2012). "Early peak temperature and mortality in critically ill patients with or without infection." Intensive Care Medicine, 38(3), 437-444. https://doi.org/10.1007/s00134-012-2478-3. Registry of 269,078 admissions in Australia and New Zealand, validated in 366,973 UK admissions; in patients without infection, risk rose above 39.0°C. (Lowest adjusted death rates at peak temperatures of 39-39.4°C.)
  6. 61.Schortgen, F., Clabault, K., Katsahian, S., Devaquet, J., et al. (2012). "Fever control using external cooling in septic shock: a randomized controlled trial." American Journal of Respiratory and Critical Care Medicine, 185(10), 1088-1095. https://doi.org/10.1164/rccm.201110-1820OC. Mortality was a secondary outcome; a confirmatory trial has been registered. (200 patients; deaths at day 14 fell from 34% to 19%.)
  7. 62.Offringa, M., Newton, R., Nevitt, S.J., and Vraka, K. (2021). "Prophylactic drug management for febrile seizures in children." Cochrane Database of Systematic Reviews, 6, CD003031. https://doi.org/10.1002/14651858.CD003031.pub4. The 2-4% figure refers to children in Great Britain and the United States; seizures recur in 30%. Across 32 randomized trials (4,431 children), intermittent ibuprofen, diclofenac or paracetamol did not prevent recurrence.
  8. 63.Trautner, B.W., Caviness, A.C., Gerlacher, G.R., Demmler, G., et al. (2006). "Prospective evaluation of the risk of serious bacterial infection in children who present to the emergency department with hyperpyrexia (temperature of 106°F or higher)." Pediatrics, 118(1), 34-40. https://doi.org/10.1542/peds.2005-2823. Rectal temperatures, 130,828 visits over two years. (20 of 103 children.)
  9. 64.Barton, A.R., Santander, C.G., Skoglund, P., Moltke, I., et al. (2025). "Insufficient evidence for natural selection associated with the Black Death." Nature, 638(8051), E19-E22. https://doi.org/10.1038/s41586-024-08496-5. Frequency of the ERAP2 C allele: 0.438 in present-day British and Irish UK Biobank participants vs 0.466 in ancient England (P=0.55). (Published 2025; the ancient English samples date from 1,000 to 2,000 years ago.)
  10. 65.Vilgalys, T.P., Klunk, J., Demeure, C.E., Cheng, X., et al. (2025). "Reply to: Insufficient evidence for natural selection associated with the Black Death." Nature, 638(8051), E23-E29. https://doi.org/10.1038/s41586-024-08497-4
  11. 66.Mecsas, J., Franklin, G., Kuziel, W.A., Brubaker, R.R., et al. (2004). "CCR5 mutation and plague protection." Nature, 427(6975), 606. https://doi.org/10.1038/427606a; Sabeti, P.C., Walsh, E., Schaffner, S.F., Varilly, P., et al. (2005). "The case for selection at CCR5-Delta32." PLoS Biology, 3(11), e378. https://doi.org/10.1371/journal.pbio.0030378. Mice lacking CCR5 showed no difference in plague survival, and denser genetic maps found the variant's pattern consistent with neutral evolution.
  12. 67.National Institute for Health and Care Excellence (2019, updated 2021). Fever in under 5s: assessment and initial management (NG143). https://www.nice.org.uk/guidance/ng143/chapter/Recommendations. Recommendations 1.2.12-1.2.13, 1.6.1-1.6.6 and 1.7.3.
  13. 68.US Food and Drug Administration (2024). Don't Overuse Acetaminophen. https://www.fda.gov/consumers/consumer-updates/dont-overuse-acetaminophen. More than 600 medications contain acetaminophen.
  14. 69.NHS (n.d.). Reye's syndrome. https://www.nhs.uk/conditions/reyes-syndrome/
  15. 70.World Health Organization (2025). Dengue and severe dengue (fact sheet, 21 August 2025). https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue. Ibuprofen and aspirin raise the risk of bleeding.
  16. 71.NHS (n.d.). High temperature (fever) in adults. https://www.nhs.uk/conditions/fever-in-adults/
  17. 72.Centers for Disease Control and Prevention (n.d.). About Sepsis. https://www.cdc.gov/sepsis/about/index.html
  18. 73.Dreier, J.W., Andersen, A.M., and Berg-Beckhoff, G. (2014). "Systematic review and meta-analyses: fever in pregnancy and health impacts in the offspring." Pediatrics, 133(3), e674-e688. https://doi.org/10.1542/peds.2013-3205. First-trimester fever was associated with 1.5 to nearly 3 times the risk of neural tube defects, heart defects and oral clefts, with some evidence that fever reducers were protective.
  19. 74.Centers for Disease Control and Prevention (n.d.). Post-travel evaluation of the ill traveler. CDC Yellow Book. https://www.cdc.gov/yellow-book/hcp/post-travel-evaluation/post-travel-evaluation-of-the-ill-traveler.html (accessed 19 September 2026)
  20. 27.Prymula, R., Siegrist, C.A., Chlibek, R., Zemlickova, H., et al. (2009). "Effect of prophylactic paracetamol administration at time of vaccination on febrile reactions and antibody responses in children: two open-label, randomised controlled trials." The Lancet, 374(9698), 1339-1350. https://doi.org/10.1016/S0140-6736(09)61208-3. Funded by GlaxoSmithKline Biologicals. Fevers above 39.5°C were uncommon in both groups. (459 infants; fevers of 38°C or more fell from 66% to 42%; antibody levels fell against all 10 pneumococcal strains in one vaccine.)
  21. 75.Llewelyn, M.J., Fitzpatrick, J.M., Darwin, E., Tonkin-Crine, S., et al. (2017). "The antibiotic course has had its day." BMJ, 358, j3418. https://doi.org/10.1136/bmj.j3418

This topic carries 75 notes in the research manuscript; the 21 above are those behind the material on this page. Full source register.