Fever in Adults and Kids: The Numbers That Matter, and When to Worry
Fever is the body fighting an infection, and most aren't dangerous. Here are the temperatures and warning signs that matter at each age, and when to get seen.
A fever is a temperature of 100.4°F (38°C) or higher, and it's usually a sign the body is fighting an infection, most often a virus. Most fevers last two to three days and aren't dangerous on their own. How worried to be depends far more on age and how the person is acting than on the number. Any fever of 100.4°F or higher in a baby under 3 months needs to be seen right away.
When a fever needs care, by age
- Under 3 months: a rectal temperature of 100.4°F or higher needs to be seen right away, day or night. Go to the ER.
- 3 to 6 months: get seen the same day for a fever of 102°F or higher, or any fever with fussiness, poor feeding or fewer wet diapers.
- 6 months to 2 years: get seen if the fever lasts more than 24 to 48 hours or comes with other symptoms.
- Older children: get seen if the fever lasts more than three days, or if they seem very unwell even after fever medicine.
- Adults: get seen for a fever of 103°F or higher, a fever lasting more than three days, or a fever with symptoms like a severe headache, rash, trouble breathing or pain when peeing.
How long do fevers last?
Fevers from common viruses usually last two to three days, and flu fevers can last up to five. A fever that lasts longer, or goes away and comes back, can mean a bacterial infection such as an ear infection, strep, a UTI or pneumonia, and is worth checking.
Taking a temperature
- Under 3 months: a rectal thermometer is the most accurate
- Young children: rectal, armpit or a temporal (forehead) thermometer
- Over 4 years and adults: under the tongue, or forehead
Bringing a fever down
- Use acetaminophen or ibuprofen, dosed by weight for children. Don't give ibuprofen to babies under 6 months, and never give aspirin to children or teens.
- Offer plenty of fluids
- Dress in light clothing, with a light blanket
- Let them rest
Treating a fever is for comfort. You don't have to bring a fever down if the person is comfortable, resting and drinking.
Fever and seizures in young children
Some children between 6 months and 5 years have a short seizure with a fever (a febrile seizure). It's frightening, but most are harmless. Lay the child on their side, don't put anything in their mouth, and time it. Call 911 if it lasts more than 5 minutes, and get seen after any first febrile seizure.
When to get seen
A clinician can find the cause of a fever and test for common culprits, such as flu, COVID and RSV, strep and UTIs. Our post on taking your child to urgent care has more for parents. At an AfterOurs clinic, a standard self-pay visit is $175, a visit with a rapid test is $250, and a self-pay in-clinic visit never costs more than $330. We contract with all major commercial and government plans, including Colorado Medicaid and Medicare. See our fever page and the cost and insurance page.
Go to the ER or call 911 if a baby under 3 months has a fever of 100.4°F or higher; if anyone with a fever has trouble breathing, a stiff neck, a severe headache, confusion or is hard to wake; has a purple or red rash that doesn't fade when pressed; has a seizure lasting more than 5 minutes; or shows signs of dehydration, such as no urine for 8 hours.
Where to find us
Four clinics across metro Denver, all ages welcome, se habla español:
- Hampden (Southeast Denver): 8 AM–11 PM daily
- Littleton: 8 AM–11 PM daily
- Thornton: 8 AM–11 PM daily
- Highlands: Mon–Fri 5–11 PM, Sat–Sun 8 AM–11 PM
Walk in, or book online.
Medically reviewed by Peter Martinuzzi, MD
This article is general health information, not medical advice for your situation. If you are worried, come in or book a telehealth visit — and if it is an emergency, call 911.