Most adults know that a fever means something’s off. But how high is too high? How long is too long? And what symptoms should make you drop everything and call a doctor? The answer isn’t just about the number on the thermometer-it’s about your body’s warning signs, your health history, and how long you’ve been feeling this way.
What Counts as a Fever?
A fever isn’t just feeling hot. Medically, it’s when your body temperature hits 100.4°F (38°C) or higher. But here’s the catch: not everyone runs at 98.6°F. Some people naturally run a little higher-maybe 99°F-and that’s normal for them. What matters isn’t the exact number, but a clear rise from your usual baseline. If you normally run at 98°F and suddenly hit 100.5°F, that’s your fever.
Doctors break fevers into tiers:
- Low-grade: 99.1°F to 100.4°F (37.3°C to 38°C)
- Moderate: 100.6°F to 102.2°F (38.1°C to 39°C)
- High-grade: 102.4°F to 105.8°F (39.1°C to 41°C)
Most fevers come from viruses-flu, colds, COVID-19, or even a stomach bug. But they can also signal bacterial infections like pneumonia or UTIs, or even non-infectious issues like autoimmune flare-ups or drug reactions. Your body raises its temperature to make it harder for germs to survive. It’s a defense, not a disease.
When to Call Your Doctor
Most fevers go away on their own in 48 to 72 hours. If yours doesn’t, it’s time to get checked-even if it’s not super high. Persistent fever is a red flag. Studies show that 90% of viral fevers resolve within three days. If yours is still going after that, you’re in the 10% where something else might be going on-maybe a bacterial infection needing antibiotics.
Here’s when to call your doctor, regardless of temperature:
- Fever lasting more than 72 hours
- Fever that keeps coming back over several days
- Fever that doesn’t drop after taking acetaminophen or ibuprofen
For most healthy adults, a fever up to 103°F (39.4°C) can be managed at home if it’s short-lived. But if it hits 103°F and keeps climbing, or doesn’t respond to medicine, don’t wait. Call your provider. That’s the threshold most major hospitals-including Penn Medicine and Hackensack Meridian Health-agree on.
When to Go to the Emergency Room
Some fever symptoms aren’t just uncomfortable-they’re dangerous. If you have a fever and any of these, go to the ER immediately:
- Temperature of 105°F (40.6°C) or higher
- Confusion, trouble waking up, or disorientation
- Seizures
- Stiff neck with headache
- Difficulty breathing or blue lips/tongue
- Severe abdominal pain
- Painful urination or unusual vaginal discharge
- A new rash that looks like purple spots or bruises
- Inability to walk, stand, or urinate
These aren’t just “maybe” signs. A stiff neck with fever could mean meningitis. A rash that doesn’t fade when you press on it could signal toxic shock syndrome. Confusion or seizures mean your brain is under stress. At 105°F, your body risks protein damage-your cells start to break down. This isn’t a “wait and see” situation.
Special Cases: Who Needs to Be More Careful
If you’re over 65, or your immune system is weakened, your fever threshold drops. You don’t need to wait for 103°F. If your temperature hits 101°F (38.3°C) or higher, call your doctor right away.
Who’s at higher risk?
- People on chemotherapy or immunosuppressants
- Those with HIV/AIDS
- Organ transplant recipients
- People taking long-term steroids
- Anyone with chronic conditions like diabetes, heart disease, COPD, or sickle cell anemia
Why? Because your body can’t fight off infections the way a healthy one can. A fever that seems mild to someone else could be the first sign of a life-threatening infection for you. In fact, CDC data from early 2023 showed that delayed fever evaluation in these groups led to 18% of preventable hospitalizations during the pandemic years.
How to Take Your Temperature Right
Not all thermometers are created equal. For adults, the most accurate reading comes from a digital oral thermometer. Place it under your tongue, close your mouth, and wait for the beep.
Forehead (temporal artery) thermometers are convenient and decently accurate-just make sure you’re not sweaty or just came in from the cold. Don’t trust armpit readings-they’re often too low.
And here’s a big mistake people make: taking their temperature right after exercise, a hot shower, or being in a hot room. That can push your reading up by 1-2°F. Wait at least 30 minutes in a cool, calm room before checking again.
What Not to Do
Don’t wait until you’re at 104°F to act. A 2023 Kaiser Permanente survey found that 42% of adults did exactly that. By then, the risk of complications is already rising.
Don’t rely on “fever reducers” to mask symptoms long-term. Taking ibuprofen or acetaminophen is fine to feel better-but if the fever keeps coming back, it’s your body screaming for help. The medicine hides the symptom, not the cause.
Don’t ignore a fever just because you “feel fine.” Some people, especially older adults, don’t feel as sick even when their fever is high. That doesn’t mean they’re okay. It means their body’s response is blunted.
What to Do While Waiting
If you’re not in emergency territory yet, here’s what to do:
- Drink water or electrolyte drinks. Fever dehydrates you fast.
- Rest. Your body needs energy to fight.
- Wear light clothing. Don’t bundle up-it traps heat.
- Use a cool washcloth on your forehead or wrists if you’re uncomfortable.
- Take acetaminophen or ibuprofen as directed-don’t double-dose.
If you have access to telehealth, many clinics now offer 24/7 virtual visits. In 2023, nearly two-thirds of non-emergency fever cases were handled remotely, saving time and avoiding crowded ERs.
Bottom Line
Fever isn’t the enemy. It’s your body’s alarm system. The problem isn’t the fever-it’s ignoring the alarm.
For most people: if it’s under 103°F and gone in 3 days, you’re probably fine. But if it’s higher, lasts longer, or comes with strange symptoms-don’t gamble. Call your doctor. Go to urgent care. Or head to the ER. The difference between waiting a day too long and acting early can mean the difference between a quick recovery and a hospital stay.
Listen to your body. And if you’re unsure? It’s always better to check.
Is a fever of 102°F dangerous for adults?
A fever of 102°F isn’t dangerous by itself for a healthy adult. It’s a moderate fever and often part of fighting off a virus. But if it lasts more than 48 hours, doesn’t respond to medicine, or comes with other symptoms like rash, confusion, or trouble breathing, you should get checked. The number matters less than how long it lasts and what else is going on.
Can stress cause a fever in adults?
Stress alone doesn’t cause a true fever-meaning a rise in core body temperature regulated by the hypothalamus. But chronic stress can weaken your immune system, making you more likely to catch an infection that then causes a fever. Some people report feeling feverish during high stress, but without a measurable temperature above 100.4°F, it’s not a medical fever.
Should I go to the ER for a fever of 104°F?
Yes. A fever of 104°F or higher is considered a medical emergency, regardless of other symptoms. At this temperature, your body is at risk of protein damage and neurological effects. Don’t wait to see if it goes down. Go to the ER or call 911 if you also have confusion, seizures, or trouble breathing.
Is it normal for a fever to come and go?
Yes, especially with viral infections like flu or COVID-19. Your temperature might spike in the evening and drop during the day. But if this pattern continues for more than 5-7 days, it’s not normal. Recurring fevers can signal bacterial infections, autoimmune issues, or even rare conditions like lymphoma. See a doctor if it keeps coming back.
Can you have a fever without being sick?
Technically, no. A fever is your body’s response to something-usually an infection, but sometimes inflammation, drugs, or even cancer. If you have a fever and no other symptoms, you still have an underlying issue. It might be early-stage, or your symptoms are hidden. Either way, it’s not normal. Get evaluated if your fever lasts more than 48 hours.
What’s the best way to treat a fever at home?
Rest, drink fluids, and take acetaminophen or ibuprofen if needed. Don’t use cold baths or alcohol rubs-they can cause shivering, which raises your temperature further. Wear light clothes, keep the room cool, and avoid bundling up. The goal isn’t to bring the fever down to normal-it’s to help you feel better while your body fights the infection.
Do I need antibiotics if I have a fever?
Not necessarily. Most fevers in adults are caused by viruses, and antibiotics don’t work on viruses. Only a doctor can determine if your fever is from a bacterial infection that needs antibiotics. Taking them unnecessarily can lead to resistance and side effects. Don’t ask for them-wait for a diagnosis.
Can a fever cause brain damage?
Fevers from infections rarely cause brain damage. But temperatures above 107.6°F (42°C) can be dangerous-and those are extremely rare outside of heatstroke. The real danger is what’s causing the fever, not the fever itself. A fever of 104°F doesn’t burn your brain. But if it’s caused by meningitis or encephalitis, those conditions can. That’s why symptoms like stiff neck, confusion, or seizures matter more than the number.