Inflammation isn’t always infection, and sepsis is the infection that can’t wait

Key takeaways

• In a 2023 study, infected goldfish swam to water 2 to 3°C warmer, giving themselves a fever, and cleared the infection in half the time of fish kept cool¹.
• CDC says at least 28% of antibiotic prescriptions in U.S. doctors’ offices and emergency rooms aren’t needed, a figure from 2014–2015 data; colds, flu and COVID-19 are viral, so antibiotics don’t help³⁷⁸.
• In 26 trials, letting a blood test called procalcitonin guide antibiotic decisions for lung and airway infections cut antibiotic use by about 2.4 days and side effects from 22% to 16%, though about half the trials had industry funding¹²¹³.
• About 1.7 million U.S. adults develop sepsis each year, and at least 350,000 die in the hospital or are discharged to hospice; CDC calls it a life-threatening emergency⁴.
• CDC’s sepsis fact sheet lists signs such as clammy skin, trouble thinking clearly, very bad pain, chills, trouble breathing and a fast heartbeat, and says to call 911 immediately if signs are severe¹⁴.

At the University of Alberta, researchers infected goldfish with a bacterium. Then they let the fish pick their own water temperature¹.

The sick fish moved to water 2 to 3°C warmer than healthy fish chose, roughly 4 to 5°F by our math. They stayed warm from the first through the eighth day after infection¹. Fish are cold-blooded, so their fever came from where they swam¹.

The fish with a fever cleared the bacteria in about 7 days. Fish held at a cooler 16°C took about 14¹. The warmth wasn’t simply cooking the germs, because in a lab dish the bacteria grew faster as the temperature rose¹. The fevered fish also had inflammation settle sooner and wounds heal better¹.

A 2015 review in Nature Reviews Immunology notes that animals with backbones have kept fever for more than 600 million years². Fever, like swelling and redness, is part of inflammation, the body’s response to harm. Infection is only one kind of harm.

Telling the two apart matters. CDC says at least 28% of antibiotic prescriptions in American doctors’ offices and emergency departments are unnecessary³.

Yet an infection that turns into sepsis can kill. About 1.7 million U.S. adults develop sepsis every year⁴.

This guide explains what inflammation is, what fever and lab tests can and can’t tell you, when antibiotics help, and which signs mean get care now.

Inflammation is the response, and infection is one cause

Around 30 B.C., the Roman writer Celsus described four signs of inflammation: heat, pain, redness and swelling². Doctors still learn them today.

Those signs come from the innate immune system, the fast, inborn layer of your defenses. Fever and swelling are among its standard tools¹⁰. Our companion article on innate and adaptive immunity explains the cells behind them.

Germs are a common trigger, but the same response can flare with no germ at all. Mayo Clinic lists causes of fever that aren’t infections, including inflammatory conditions such as rheumatoid arthritis, heat exhaustion, some medicines, some vaccines and certain tumors⁵.

The overlap can fool even blood tests. In a small study of people with a blood clot in the lung and a fever, common inflammation markers often rose, though the clot isn’t an infection¹¹. That’s why doctors look at the whole picture, not one number.

Long-lasting, low-grade inflammation is a separate problem. Our articles on anti-inflammatory claims and on inflammation and aging cover it. This guide sticks to the short-term kind.

Fever is a defense with a 600-million-year track record

Fever raises core body temperature by about 1 to 4°C². It’s costly: each 1°C rise takes 10% to 12.5% more energy². A habit that expensive, kept for so long, hints that it pays off.

Animal studies suggest it does. When researchers kept infected desert iguanas from warming themselves by about 2°C, survival fell by 75%². In rabbits infected with a virus called rinderpest, 70% of those given aspirin died, compared with 16% of rabbits allowed a normal fever².

Fever-range heat seems to sharpen immune cells. In lab and animal work, it boosted neutrophils’ germ-killing burst and the killing power of natural killer cells. It helped dendritic cells travel to lymph nodes, and it roughly doubled how well lymphocytes latch onto lymph node vessels².

The same review flags limits. Uncontrolled fever is tied to worse outcomes in sepsis and brain injury². And most of this evidence comes from animals and lab work, not trials in sick people².

The American Academy of Pediatrics (AAP) offers reassurance: in most cases, a fever means the body’s defenses are at work against an infection⁶.

What counts as a fever? For babies, the AAP uses a rectal temperature of 100.4°F (38°C) or higher, with the same cutoff for mouth, ear and forehead-artery readings⁶. Mayo Clinic sets a slightly lower line for oral readings, 100°F (37.8°C)⁵.

Lowering a fever is mostly about comfort. CDC lists fever reducers with pain relievers, saline drops, warm compresses, fluids and rest as ways to help a sick child feel better⁹. Our article “Chicken soup won’t cure a cold, and comfort still counts until it delays care” explains why pediatricians treat the child, not the number.

When a fever needs a call

The thermometer matters most at the youngest ages.

• Babies 3 months or younger: The AAP says to call the pediatrician immediately for a rectal temperature of 100.4°F (38°C) or higher, even if the baby shows no other sign of illness⁶.
• Older babies and children: Sources differ. The AAP’s 2025 guidance says you may need to call at 101°F (38.3°C) for babies 3 to 6 months and 103°F (39.4°C) after 6 months⁶. Mayo Clinic sets its line for babies 3 to 6 months at above 102°F (38.9°C)⁵.
• Behavior counts, too: Mayo Clinic lists listlessness, confusion, poor eye contact, repeated vomiting and a fever lasting more than three days as reasons to call about a child⁵.
• Adults: Mayo Clinic says to call for 103°F (39.4°C) or higher. Get immediate care for a fever with a severe headache, rash, unusual sensitivity to light, a stiff neck, confusion, persistent vomiting, trouble breathing, chest pain, belly pain, pain when urinating, or a seizure⁵.

Viruses don’t respond to antibiotics

CDC says antibiotics are only for some bacterial infections⁹. Its chart of common respiratory illnesses sorts them like this⁷:

• Colds, flu, COVID-19 and sore throats other than strep: caused by viruses; antibiotics won’t help.
• Bronchitis or chest colds in otherwise healthy people: antibiotics aren’t needed.
• Middle ear and sinus infections: caused by viruses or bacteria; antibiotics sometimes help.
• Strep throat and whooping cough: caused by bacteria; antibiotics are needed.

Urinary tract infections also need antibiotics, CDC adds⁹.

How common is the misuse? A CDC-led study found that unnecessary prescriptions in American doctors’ offices and emergency departments barely fell, from 30% in 2010–2011 to 28% in 2014–2015⁸. Children improved a lot, from 32% to 19%. Adults didn’t improve at all⁸.

CDC still cited the at-least-28% figure in materials updated in 2026³. It adds that the prescriptions likely needed still leave room to improve the drug choice, dose and length³.

The harms are real. Antibiotic reactions cause 1 in 5 emergency visits for medicine side effects, and in children they’re the top cause⁹.

Side effects range from nausea, diarrhea, yeast infections and rash to C. diff, a gut infection that can cause severe colon damage and death⁷⁹. Any use can also feed antibiotic resistance³.

CDC says to get help right away for severe diarrhea, which could be C. diff, or for a severe allergic reaction such as wheezing, hives or trouble breathing⁷.

There are middle paths. For a mild ear infection, a clinician may suggest watching a child for 2 to 3 days first, and CDC notes that two in three children with mild ear infections get better without antibiotics⁷. With delayed prescribing, you get a prescription but fill it only if symptoms don’t improve or get worse after a set time⁷.

Lab tests offer clues, not verdicts

White blood cell count. Neutrophils, the most common white cells, mainly fight bacteria and fungi. So in a viral infection the white count usually isn’t high and may even be low, the Immune Deficiency Foundation explains¹⁰. That’s a hint, not proof.

C-reactive protein (CRP). CRP rises with inflammation, driven by the immune system’s chemical messengers¹¹. It goes up with infections, but also with causes that aren’t infections, as the lung-clot study showed¹¹. Our article on anti-inflammatory claims explains the heart-risk version of this test.

Procalcitonin. This protein is almost undetectable in healthy blood. Under major stress, such as body-wide inflammation, sepsis or burns, many organs start making it¹¹. Viral infections tend to keep it low, according to a review for emergency doctors¹³. It can also rise in some thyroid and hormone-making cancers¹¹.

It isn’t perfect. In a study of 69 people with lupus who had a bacterial infection, 43 had sepsis and 11 died. Their CRP rose sharply, but their procalcitonin didn’t¹¹.

The best evidence for procalcitonin is about using it to guide antibiotics for lung and airway infections. A pooled analysis of 26 trials in 12 countries, with 6,708 patients, compared procalcitonin-guided care with usual care¹².

Outcome	Procalcitonin-guided	Usual care
Days on antibiotics	5.7	8.1
Antibiotic side effects	16%	22%
Deaths within 30 days	286 of 3,336 (about 8.6%)	336 of 3,372 (about 10.0%)

By our math, that’s about 14 fewer deaths and 60 fewer people with side effects per 1,000 patients¹². The analysis was funded by Britain’s National Institute for Health Research¹².

Read it with care. Most trials took place in hospital wards, emergency departments or intensive care units; only 2 were in primary care¹³. About half the trials had funding from companies that make such tests, the emergency-medicine review noted, and it said the lead author’s industry funding broke Cochrane’s sponsorship rules¹³.

Sepsis: when the body’s defense becomes the danger

CDC describes sepsis as an infection that overwhelms the body’s ability to fight it, until the body itself can start shutting down¹⁴. CDC calls it a life-threatening medical emergency⁴.

Each year about 1.7 million U.S. adults and more than 18,000 children develop sepsis. At least 350,000 adults and more than 1,800 children die in the hospital or are discharged to hospice⁴. By our math, that’s roughly 1 in 5 adults.

Most cases start before the person ever reaches a hospital, CDC says. The infections behind it most often begin in the gut, lungs, skin or urinary tract, and most people who develop sepsis have at least one other medical condition or a recent hospital stay⁴.

Speed matters. CDC tells health care providers to treat sepsis as soon as possible⁴. A 2017 study in the New England Journal of Medicine tracked 49,331 sepsis patients at 149 New York hospitals. Each extra hour before antibiotics was linked to about 4% higher odds of dying in the hospital¹⁵.

That study was observational, so it shows a link rather than proof, but it points the same way as CDC’s advice⁴¹⁵. There’s no one-number answer at the bedside, either. CDC says a clinician makes the diagnosis through a medical assessment that draws on several tests, such as blood cultures⁴.

Inflammation or infection? The sorting guide

This table pulls official guidance into one place. It’s a starting point for a conversation with a clinician, not a diagnosis.

What you notice	Likely cause	Will an antibiotic help?	When to get care
Runny nose, sore throat, cough, mild fever	Usually a virus: cold, flu or COVID-197	No7	Most respiratory viruses clear in a week or two9. Adults: call for 103°F or higher; get immediate care for trouble breathing or chest pain5
Chest cold (bronchitis) in an otherwise healthy person	CDC lists it among illnesses that don’t need antibiotics7	No7	Immediate care for fever with trouble breathing or chest pain5
Ear or sinus pain	Virus or bacteria7	Sometimes; mild ear infections may be watched 2 to 3 days7	Fever with a severe headache, stiff neck or light sensitivity needs immediate care5
Diagnosed strep throat, whooping cough or urinary tract infection	Bacteria79	Yes79	Fever with pain when urinating needs immediate care5
Fever or swelling during a flare of a known condition, such as rheumatoid arthritis	May be inflammation without infection, but infections also occur and can be hard to spot511	Only if a clinician finds a bacterial infection9	Adults: call for 103°F or higher or any red-flag sign above5
Fever in a baby 3 months or younger	Unknown until checked	A clinician decides	100.4°F rectal or higher: call the pediatrician immediately6
Infection plus confusion, trouble breathing, racing heart, clammy skin, very bad pain or shivering	Possible sepsis4	Clinicians decide, as soon as possible4	Call 911 for severe signs14

How strong is the evidence?

Claim	What the research shows	Evidence level
Antibiotics help colds, flu, COVID-19 or chest colds in healthy people	CDC says antibiotics aren’t needed for these and can cause harm79	Evidence says no, or unsafe
Antibiotics treat strep throat, whooping cough and urinary tract infections	CDC lists them as needed79	Strong evidence
Watching a mild ear infection for 2 to 3 days first	Two in three children recover without antibiotics7	Strong evidence
Procalcitonin-guided care trims antibiotic use in airway infections	26 trials; 2.4 fewer days and fewer side effects; about half had industry funding1213	Strong evidence
Fever helps the body fight infection	Consistent in animals from fish to rabbits; mostly animal and lab evidence12	Promising but early
Sepsis needs treatment as fast as possible	CDC guidance, plus a study of 49,331 patients tying each hour of antibiotic delay to about 4% higher odds of death; the study was observational415	Strong evidence

Trusting the body, and knowing when to hurry

Natural-health traditions trust the body to heal, and for most inflammation that trust is earned. Fever is an ancient defense². Most respiratory viruses go away in a week or two without treatment⁹. The comfort steps CDC itself suggests are rest, fluids, saline and warm compresses⁹.

Our articles on comfort care and on gargles and steam weigh those home remedies, including which ones carry risks. Skipping an antibiotic you don’t need is a choice natural-health readers can feel good about, and our companion article on the microbiome looks at what broad antibiotics do to gut bacteria.

Trust has limits, though. A young baby with a fever, an infection that keeps getting worse, or sudden confusion and trouble breathing are the body’s own alarms. Listening to them, and getting care fast, is part of respecting how the body works.

Questions readers ask

The bottom line

Inflammation is your body defending itself, and infection is only one reason it happens. Fever, swelling and redness are usually signs of a system at work, and most viral illnesses clear without antibiotics, which can cause real harm when they aren’t needed.

The exceptions are urgent. A baby 3 months or younger with a fever, an infection that keeps getting worse, or signs like confusion, trouble breathing and a racing heart need care now. Sepsis is a medical emergency, and calling 911 for severe signs is the natural, sensible response.

Keep exploring: vitamin A measles toxicity.

Related reading

• Your immune system has two layers, and the 2025 Nobel went to its brakes
• Chicken soup won’t cure a cold, and comfort still counts until it delays care
• Plain water gargles cut colds in one trial, while steam bowls have burned children
• Inflammation can harm your heart, but an “anti-inflammatory” label proves little
• The symptoms that mean call 911 now

Sources

1. Haddad F, et al. “Fever Integrates Antimicrobial Defences, Inflammation Control, and Tissue Repair in a Cold-Blooded Vertebrate.” eLife, 2023;12:e83644. pubmed.ncbi.nlm.nih.gov
2. Evans SS, Repasky EA, Fisher DT. “Fever and the Thermal Regulation of Immunity: The Immune System Feels the Heat.” Nature Reviews Immunology, 2015;15(6):335–349 (NIH author manuscript). pmc.ncbi.nlm.nih.gov
3. Centers for Disease Control and Prevention. “Improve Outpatient Antibiotic Use” (communication resources). CDC, published March 6, 2026, updated September 17, 2026. cdc.gov
4. Centers for Disease Control and Prevention. “About Sepsis.” CDC, updated August 17, 2026. cdc.gov
5. Mayo Clinic Staff. “Fever: Symptoms and Causes.” Mayo Clinic, December 24, 2025. mayoclinic.org
6. American Academy of Pediatrics. “Fever and Your Baby.” HealthyChildren.org, last updated September 23, 2025 (adapted from Caring for Your Baby and Young Child: Birth to Age Five, 8th ed., 2024). healthychildren.org
7. Centers for Disease Control and Prevention. “Know When Antibiotics Work” (patient handout CS350217-C). CDC, undated. cdc.gov
8. Hersh AL, King LM, Shapiro DJ, Hicks LA, Fleming-Dutra KE. “Unnecessary Antibiotic Prescribing in US Ambulatory Care Settings, 2010–2015.” Clinical Infectious Diseases, 2021;72(1):133–137 (CDC Stacks record). stacks.cdc.gov
9. Centers for Disease Control and Prevention. “Antibiotics Aren’t Always the Answer” (patient handout). CDC, undated. stacks.cdc.gov
10. Immune Deficiency Foundation. “How Your Immune System Works” (adapted from the IDF Patient & Family Handbook for Primary Immunodeficiency Diseases, 6th ed., 2019). Accessed October 2026. primaryimmune.org
11. Mandell BF. “Infection or Not Infection, That Is the Question—Is Procalcitonin the Answer?” Cleveland Clinic Journal of Medicine, 2019;86(5):297–298. doi.org
12. Schuetz P, et al. “Effect of Procalcitonin-Guided Antibiotic Treatment on Mortality in Acute Respiratory Infections: A Patient Level Meta-Analysis.” Lancet Infectious Diseases, 2018 (published online October 18, 2017; University of Fribourg repository record). pubmed.ncbi.nlm.nih.gov
13. Storch B (edited by Swaminathan A). “Procalcitonin Utility in Acute Respiratory Illnesses.” Core EM, August 9, 2018. coreem.net
14. Centers for Disease Control and Prevention. “Sepsis Is an Emergency” (simplified patient fact sheet). CDC, undated. cdc.gov
15. Seymour CW, Gesten F, Prescott HC, et al. “Time to Treatment and Mortality during Mandated Emergency Care for Sepsis.” New England Journal of Medicine, 2017;376(23):2235-2244 (Washington University in St. Louis research record). profiles.wustl.edu