Fever in adults: causes, treatment by video and when to worry

In adults, a temperature by mouth above 99 °F to 99.5 °F (37.2 °C to 37.5 °C), depending on the time of day, is likely a fever. A fever is not an illness in itself; it is a sign your body is reacting to something, usually an infection.

Use a digital thermometer and write down each reading with the time. That record helps the doctor a lot during your visit.

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Most fevers come from viral or bacterial infections. The causes table on this page lists the common ones. Some clues that help tell them apart:

• Sudden fever with muscle aches and exhaustion: possibly flu.
• Fever with cough, sore throat or loss of smell: possibly COVID-19.
• Fever with burning when you pee or back pain: possibly a urinary tract infection reaching the kidneys, which needs prompt care.
• Fever with pain behind the eyes, bone pain or a rash: think about dengue.
• Fever with a very painful throat, or with congestion and facial pain: possibly a throat or sinus infection.

Chills are common while a fever is rising. MedlinePlus advises not to bundle up someone who has chills and not to use ice or cold water, because shivering raises the body temperature; take off extra clothes and blankets instead. If fever and chills come with burning when you pee or pain in the side of your back, it can be a kidney infection: read UTI treatment and get checked soon.

Dengue is a cause of fever you cannot ignore on the island. Puerto Rico's Department of Health declared a dengue public health emergency in 2024; the CDC counted 6,291 cases that year, and more than half of patients were hospitalized.

Dengue warning signs usually start 24 to 48 hours after the fever goes away, so a falling fever does not mean you are in the clear. If dengue is possible, the CDC advises acetaminophen for fever and pain and no aspirin or ibuprofen. Read our dengue warning signs guide, which matters for visitors as much as residents.

If you feel reasonably well and have no warning signs:

• Rest and drink fluids often: water or drinks with electrolytes.
• Acetaminophen or ibuprofen lower fever in adults. Follow the label, and do not combine products that contain the same ingredient.
• If dengue is possible, use acetaminophen only.
• Check your temperature two or three times a day and write it down.
• Stay home and wash your hands often so you do not spread the infection.
• If a fever lasts more than 48 to 72 hours, keeps coming back, or comes with urinary symptoms, the doctor will decide whether you need blood work or a urine test and give you an order for a lab near you.
• Many viral fevers only need home care, and antibiotics do not work on viruses. The doctor will explain what your situation needs.

For your safety, call 911 or go to an emergency room if a fever comes with confusion, being hard to wake, a severe headache with a stiff neck, trouble breathing, blue lips or nails, a seizure, or dengue warning signs. Get care soon if your fever reaches 103 °F (39.4 °C) or higher and stays there, lasts more than 48 to 72 hours, or comes with a new rash or bruising, or if you have a serious medical condition or a weakened immune system. More in when to go to the ER.

We see children of all ages, but fever in children is judged differently depending on age. For their safety, a baby under 3 months with a fever needs the emergency room. For patients under 21, a parent or legal guardian stays on camera for the whole visit and shows photo ID. Read our page on fever in children.