Overview
A fever is when your body temperature goes up for a while because of an illness. It's one of the ways your body fights germs. Most germs grow best at normal body temperature, 98.6°F (37°C), so a fever can help slow them down.
Your normal temperature changes during the day. It's usually highest in the evening. Exercise, heavy clothes, hot weather, and some medicines can also raise it a little. A fever from an infection rarely goes above 105°F (40.6°C) unless the person is too bundled up or in a hot place.
When to get medical care
Causes
Most fevers come from infections caused by viruses or bacteria. Vaccines can cause a low fever for a day or two. Less often, a fever comes from inflammation, a blood clot, some medicines, or cancer. A cold or other virus can cause a fever of 102°F to 104°F (38.9°C to 40°C) without being serious. Teething may raise a baby's temperature a little, but not above 100°F (37.8°C).
OwieClear has pages on 114 conditions that can cause fever. Each shows how soon to get care.
Common causes
- Influenza (flu)See a doctor if it doesn't get better
A viral infection that tends to come on suddenly, with fever, body aches, and tiredness.
- COVID-19See a doctor if it doesn't get better
A viral infection that can cause fever, cough, tiredness, and loss of taste or smell.
- DiverticulitisOften needs care today
Inflamed or infected small pouches in the wall of the colon.
- Fever in a newborn (under 3 months)May need emergency care
A rectal temperature of 100.4°F (38°C) or higher in a baby under 3 months always needs a doctor right away, because young babies can't fight infections well…
- Strep throatSee a doctor if it doesn't get better
A bacterial throat infection.
- Fever after international travel (such as malaria or dengue)Often needs care today
A fever within weeks or months after traveling, especially to parts of Africa, Asia, or Latin America, can be malaria, dengue, or another travel infection that…
- Fever with no clear cause (fever of unknown origin)Often needs care today
A fever that keeps coming back or lasts more than 3 weeks without a clear cause after basic tests.
- Kidney infectionOften needs care today
A bladder infection that has spread up to the kidneys.
- MalariaOften needs care today
A serious infection spread by mosquitoes in parts of Africa, Asia, and Latin America.
- Mononucleosis (mono)See a doctor if it doesn't get better
A viral infection that causes extreme tiredness, sore throat, fever, and swollen glands.
- PneumoniaOften needs care today
An infection that inflames the air sacs in the lungs.
- MeaslesOften needs care today
A very contagious virus that starts with high fever, cough, runny nose, and red, watery eyes, then a rash that begins at the hairline and spreads down the body.
- MeningitisMay need emergency care
An infection of the lining around the brain and spinal cord.
- Serious drug rash (DRESS or AGEP)May need emergency care
A serious reaction to a medicine, usually 2 to 8 weeks after starting it (for DRESS), with a spreading rash, fever, face swelling, and swollen glands.
- Spine infection (vertebral osteomyelitis or discitis)Often needs care today
An infection of the bones or discs of the spine, often spread through the blood from a skin, urine, or heart infection, or after spine surgery.
- Sweet syndrome (sudden painful red bumps with fever)Often needs care today
Sudden fever with tender, red or purple raised bumps or plaques on the arms, face, neck, or upper body.
- Tick fever (such as Rocky Mountain spotted fever)Often needs care today
Infections from tick bites that cause fever, headache, body aches, and sometimes a rash of small spots that often starts on the wrists and ankles.
Other possible causes
- High inflammation markers (CRP or ESR)See a doctor if it doesn't get better
CRP and ESR are blood tests that rise with inflammation anywhere in the body.
- Side effects after a vaccine (including the COVID-19 shot)Often treated at home
After a vaccine it's common to have a sore, red, or swollen arm, and for a day or two to feel tired, achy, or feverish, or to have a headache or swollen glands…
- TonsillitisSee a doctor if it doesn't get better
Inflammation of the tonsils, usually from a virus or strep.
- Viral gastroenteritis (stomach bug)Often treated at home
A viral infection of the stomach and intestines that causes vomiting and diarrhea.
- Viral rash (viral exanthem)Often treated at home
A widespread rash of small pink or red spots that comes with or after a viral illness, very common in children.
- Aspiration pneumonia (inhaling food or vomit)Often needs care today
Lung inflammation or infection from breathing in food, drink, vomit, or oily substances.
- CroupSee a doctor if it doesn't get better
A viral infection that swells the voice box and windpipe, causing a barking cough like a seal and a hoarse voice, often worse at night.
- Cytomegalovirus (CMV)See a doctor if it doesn't get better
A common virus that usually causes no or mild symptoms.
- Ear infectionSee a doctor if it doesn't get better
An infection behind the eardrum, often after a cold.
- Fever seizure in a child (febrile seizure)Often needs care today
A seizure caused by a fever in a young child, usually between 6 months and 5 years old.
- Food poisoningOften treated at home
Illness from eating contaminated food, causing nausea, vomiting, and diarrhea, often within hours.
- Genital herpesSee a doctor if it doesn't get better
A common viral STI that causes painful blisters or sores on the genitals.
- Hand, foot, and mouth diseaseOften treated at home
A common viral illness in young children that causes fever, painful mouth sores, and a spotty rash on the hands and feet.
- HerpanginaOften treated at home
A viral infection in young children, related to hand, foot, and mouth disease, causing sudden fever, sore throat, and small blisters or sores at the back of…
- Infected surgical wound (surgical site infection)Often needs care today
An infection where surgery was done, usually within 30 days.
- Low white blood cell count (neutropenia)Often needs care today
Too few infection-fighting white blood cells.
- MastitisOften needs care today
An inflamed or infected breast, most often while breastfeeding.
- RoseolaOften treated at home
A viral illness in young children with a few days of high fever, followed by a pink rash after the fever goes away.
- Tooth abscess (tooth infection)Often needs care today
A pocket of infection at a tooth's root or in the gum.
- Anal abscess or fistulaOften needs care today
A painful, swollen, warm lump near the anus from an infected gland, sometimes with fever or pus.
- AppendicitisOften needs care today
Inflammation of the appendix, often starting near the belly button and moving to the lower right side.
- Bile duct infection or inflammation (cholangitis)Often needs care today
Infection or inflammation of the bile ducts.
- Blood transfusion reactionOften needs care today
A reaction during or after receiving blood.
- Bone infection (osteomyelitis)Often needs care today
An infection in a bone, from the blood, a nearby wound (like a diabetic foot ulcer), or after surgery or a broken bone.
- C. diff infectionOften needs care today
Diarrhea, belly cramps, and sometimes fever during or after taking antibiotics, from germs that take over the gut.
- Cat scratch diseaseSee a doctor if it doesn't get better
An infection from a cat scratch or bite, often from a kitten.
- CellulitisOften needs care today
A bacterial skin infection that makes skin red, warm, swollen, and painful.
- ChickenpoxSee a doctor if it doesn't get better
A contagious viral illness that causes an itchy rash of spots that turn into blisters and then scab, often with fever.
- EpididymitisOften needs care today
Swelling of the tube behind the testicle, often from an infection such as an STI.
- Fungal lung infection (valley fever, histoplasmosis, blastomycosis, or aspergillosis)See a doctor if it doesn't get better
Infections from breathing in fungus spores from soil, bird or bat droppings, or decaying plants.
- HIVSee a doctor if it doesn't get better
A virus that weakens the immune system.
- Hypersensitivity pneumonitis (farmer's lung, bird fancier's lung)See a doctor if it doesn't get better
An allergic reaction in the lungs to breathed-in dust from birds, moldy hay, hot tubs, humidifiers, or certain work materials.
- Joint infection (septic arthritis)Often needs care today
A bacterial infection inside a joint.
- Living without a working spleen (asplenia)Often needs care today
Having no spleen, from surgery or from diseases like sickle cell that damage it.
- Lung abscess or pus around the lung (empyema)Often needs care today
A pocket of pus in the lung or in the space around it, usually after pneumonia, inhaling vomit or food, or poor dental health.
- LupusSee a doctor if it doesn't get better
An autoimmune condition where the immune system attacks the body's own tissues.
- Lyme diseaseSee a doctor if it doesn't get better
An infection from a tick bite.
- MpoxSee a doctor if it doesn't get better
A virus that causes a rash of bumps that turn into blisters and scabs, often on the genitals, anus, or mouth, along with fever, swollen glands, and aches.
- Panniculitis (inflamed fat under the skin)See a doctor if it doesn't get better
Tender, red lumps deep under the skin from inflammation of the fat layer, often on the shins or thighs.
- Pelvic inflammatory disease (PID)Often needs care today
An infection of the uterus, fallopian tubes, or ovaries, often from an untreated STI.
- Pericarditis (inflamed sac around the heart)Often needs care today
Inflammation of the thin sac around the heart, often after a virus.
- PeritonitisMay need emergency care
Inflammation or infection of the lining of the belly, often from a burst appendix, ulcer, or bowel.
- Problems after a butt lift or butt injectionsOften needs care today
Problems after a Brazilian butt lift (BBL), butt implants, or butt injections.
- Problems with a joint replacementOften needs care today
Pain, swelling, instability, or new clicking in a replaced hip or knee, which can come from wear, loosening, infection, or dislocation.
- ProstatitisOften needs care today
Swelling or infection of the prostate.
- Salmonella from pets (reptiles or backyard chickens)See a doctor if it doesn't get better
Turtles, lizards, snakes, frogs, and backyard chickens and ducks often carry salmonella, even when they look healthy.
- Scarlet feverSee a doctor if it doesn't get better
A strep throat infection with a red, rough rash that feels like sandpaper, and sometimes a red strawberry tongue.
- SepsisMay need emergency care
The body's extreme response to an infection, which can damage organs and be deadly.
- Sudden confusion in an older adult (delirium)Often needs care today
A sudden change in thinking, like confusion, seeing things, being unusually sleepy or agitated, or not acting like themselves.
- Transplant rejection and transplant problemsOften needs care today
The immune system attacking a transplanted organ, or after a stem cell transplant, donor cells attacking the body (graft-versus-host disease).
- ADEM (brain and spinal cord inflammation after infection)May need emergency care
A sudden, widespread attack of inflammation in the brain and spinal cord, usually a week or two after a viral infection, mostly in children.
- AnthraxMay need emergency care
A rare, serious infection from bacteria found in soil and animals.
- BabesiosisOften needs care today
A tick-borne infection of the red blood cells, mostly in the Northeast and upper Midwest.
- Bird flu (avian influenza)Often needs care today
A flu virus that spreads among birds and sometimes dairy cows, and rarely to people who handle sick animals or raw milk.
- BrucellosisSee a doctor if it doesn't get better
An infection from unpasteurized milk or cheese, or contact with infected animals.
- Chagas diseaseSee a doctor if it doesn't get better
An infection from a parasite spread by kissing bugs, mostly in Latin America.
- Deep neck abscess (retropharyngeal or parapharyngeal)May need emergency care
A pocket of pus deep in the neck behind or beside the throat, mostly in young children after a throat infection, or in adults after dental infections.
- DiphtheriaMay need emergency care
A serious bacterial infection, now rare in the US because of vaccines.
- Ebola and other viral hemorrhagic feversMay need emergency care
Rare, severe viral infections found mainly in parts of Africa.
- Encephalitis (inflamed brain)May need emergency care
Inflammation of the brain, usually from a virus such as herpes or West Nile.
- EpiglottitisMay need emergency care
Swelling of the flap at the top of the windpipe, which can quickly block breathing.
- HantavirusMay need emergency care
A rare, serious infection from breathing dust contaminated by rodent droppings or urine.
- HLH (immune system overreaction)May need emergency care
A life-threatening condition where immune cells become overactive and attack the body's own blood cells and organs.
- Immune deficiency (including SCID)Often needs care today
Conditions where the immune system doesn't work well, causing frequent, severe, or unusual infections.
- Kawasaki diseaseOften needs care today
A rare illness in young children that causes a fever lasting 5 days or more, along with red eyes, red cracked lips, a strawberry tongue, rash, and swollen…
- Legionnaires' diseaseOften needs care today
A serious pneumonia from breathing in water mist containing Legionella bacteria, from hot tubs, cooling towers, or plumbing.
- LeishmaniasisSee a doctor if it doesn't get better
A parasite infection spread by sand fly bites in tropical and desert areas.
- LeptospirosisOften needs care today
A bacterial infection from water or soil contaminated by animal urine, often after floods, swimming in fresh water, or contact with rats.
- Liver abscessMay need emergency care
A pocket of pus in the liver from bacteria (often spreading from the gallbladder or bowel) or from amoebas after travel.
- Malignant hyperthermia (reaction to anesthesia)May need emergency care
An inherited, life-threatening reaction to certain anesthesia gases and the muscle relaxant succinylcholine.
- Malignant otitis externa (spreading ear canal infection)May need emergency care
A serious ear canal infection that spreads into the bone at the base of the skull, almost always in older adults with diabetes or a weak immune system.
- MastoiditisOften needs care today
An infection of the bone behind the ear, usually after an ear infection.
- MumpsSee a doctor if it doesn't get better
A contagious virus that causes puffy, sore cheeks and jaw from swollen salivary glands, along with fever, headache, and tiredness.
- Parrot fever (psittacosis)See a doctor if it doesn't get better
A lung infection spread by pet birds like parrots, parakeets, and cockatiels, or pigeons.
- Periodic fever syndromes (including FMF)See a doctor if it doesn't get better
Inherited conditions that cause repeated attacks of fever with belly, chest, or joint pain or rash, lasting days, then feeling fine between.
- PlagueMay need emergency care
A rare bacterial infection spread by fleas from rodents, found in parts of the western US.
- PolioOften needs care today
A virus that can cause paralysis, now prevented by vaccines.
- Q feverSee a doctor if it doesn't get better
A bacterial infection from breathing in dust contaminated by farm animals, especially during birthing of sheep, goats, or cattle.
- Reye's syndromeMay need emergency care
A rare, serious illness causing brain swelling and liver damage in children and teens, linked to taking aspirin during a viral illness like flu or chickenpox.
- Rheumatic feverOften needs care today
An illness that can follow untreated strep throat, causing fever, painful joints that move from one joint to another, rash, jerky movements, and inflammation…
- Smallpox and related poxvirusesMay need emergency care
A deadly virus that was eliminated worldwide in 1980.
- Staph scalded skin syndromeMay need emergency care
A toxin from staph bacteria that makes the skin red, tender, and peel in sheets like a burn, mostly in babies and young children.
- Toxic megacolonMay need emergency care
A dangerous swelling of the colon, usually during a severe flare of ulcerative colitis or C.
- Toxic shock syndromeMay need emergency care
A rare, life-threatening reaction to bacterial toxins, linked to tampons, wounds, or surgery.
- Tuberculosis (TB)See a doctor if it doesn't get better
A bacterial infection that usually affects the lungs.
- Vasculitis (inflamed blood vessels)Often needs care today
A group of conditions where the immune system inflames blood vessels.
- West Nile virusSee a doctor if it doesn't get better
A virus spread by mosquitoes.
- Yellow feverOften needs care today
A virus spread by mosquitoes in parts of Africa and South America.
- Zika virusSee a doctor if it doesn't get better
A virus spread mainly by mosquitoes in tropical areas, and also through sex.
Less likely causes
- Common coldOften treated at home
A mild viral infection of the nose and throat.
- Boil or skin abscessSee a doctor if it doesn't get better
A painful, red, swollen lump filled with pus, usually from a bacterial infection of a hair follicle.
- Rheumatoid arthritisSee a doctor if it doesn't get better
An autoimmune condition where the immune system attacks the joints.
- Sinus infection (sinusitis)Often treated at home
Swelling of the sinuses, often after a cold, that causes facial pressure and a stuffy nose.
- Rodent exposure (bites or droppings)See a doctor if it doesn't get better
Rat and mouse bites can cause infection or rat-bite fever.
- ShinglesOften needs care today
A painful rash, usually in a band on one side of the body, caused by the chickenpox virus reawakening.
- Skin infections and wounds from injectingOften needs care today
Injecting drugs can cause abscesses, cellulitis, and track marks.
- SyphilisSee a doctor if it doesn't get better
A bacterial STI that often starts as a painless sore.
What you can do at home
- Rest and drink plenty of fluids, like water, soup, ice pops, or gelatin. Don't give young children a lot of juice, and don't give them sports drinks.
- You don't always need to treat a fever. If the person is comfortable, eating and drinking, and acting normal, rest may be enough. Treat it if they're uncomfortable, throwing up, dehydrated, or can't sleep. The goal is to bring it down, not to make it normal.
- Dress in one light layer and use one light blanket. Don't bundle up someone who has chills. A fan may help.
- A lukewarm bath or sponge bath can help, best after medicine. Don't use cold baths, ice, or rubbing alcohol. They cause shivering, which can raise the temperature.
- Don't force food. Eating is fine if the person wants to.
Medicines
Acetaminophen (Tylenol) and ibuprofen (Advil, Motrin) both lower fever in children and adults. Follow the package directions and dose by weight. Don't give ibuprofen to babies 6 months or younger. Don't give aspirin to children unless their doctor says to. For babies 3 months or younger, call the doctor before giving any medicine.
At the doctor
The doctor will try to find what's causing the fever. A child who still wants to play, eats and drinks well, is alert and smiling, and looks well once the fever comes down probably doesn't have anything serious.
Questions you may be asked
- How high the fever has been, and how you took the temperature
- How long it has lasted, and whether it comes and goes
- Other symptoms, like cough, sore throat, earache, rash, or pain when peeing
- Recent vaccines, travel, or contact with sick people
- Long-term health problems and medicines
The exam
They'll check the skin, eyes, ears, nose, throat, neck, chest, and belly.
Tests
- Blood tests, like a complete blood count (CBC)
- A urine test (urinalysis)
- A chest X-ray
Check your symptoms
Most symptoms have more than one possible cause. The symptom checker looks at all of your symptoms together, plus your age, and shows how soon to get care.
Sources
Last updated 2026-11-13.