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Fever in adults: possible causes and when to worry

Is this an emergency?

This is not an emergency service. Call 911 or go to the nearest emergency room if you have:

  • Confusion, unusual sleepiness or being hard to wake
  • Severe headache with a stiff neck
  • Trouble breathing, or bluish lips, tongue or nails
  • A seizure
  • Dengue warning signs: belly pain, vomiting 3 or more times in 24 hours, bleeding from the nose or gums, blood in vomit or stool, or feeling extremely tired or restless
  • A fever of 105 °F (40.6 °C) or higher that does not come down with treatment

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Warning signs: when to go to the ER

A fever in adults is usually the body fighting an infection such as flu, COVID-19 or a urinary tract infection, and in Puerto Rico dengue belongs on that list too. Rest, drink fluids and watch for warning signs. Go to the ER if a fever comes with confusion, a stiff neck, trouble breathing or dengue warning signs such as belly pain or bleeding gums.

  • Flu (influenza) — sudden fever, muscle aches, exhaustion and cough
  • COVID-19 — fever with cough, sore throat or loss of smell or taste
  • Urinary tract infection — burning when you pee; with fever or back pain it may involve the kidneys
  • Dengue — fever with eye, bone or joint pain; watch for warning signs as the fever falls
  • Sore throat — fever with pain on swallowing
  • Sinus infection — fever with congestion and facial pain

What counts as a fever in an adult?

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 great deal if you book a video visit.

Common causes of fever in adults

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: see sore throat.

Fever and dengue in Puerto Rico

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.

Home care for a fever

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.

When can a telehealth visit help with a fever?

An online doctor visit can help when you have a fever with flu, COVID-19, sinus or urinary symptoms and no warning signs, or when a fever has lasted more than 48 to 72 hours and you want to know what to do next. The doctor will assess your symptoms and whether you need a test or an in-person visit. If appropriate, the doctor can prescribe medication. We do not prescribe controlled substances.

A visit costs US$79 and you must be physically in Puerto Rico during the video call.

When do you need the ER or an in-person visit?

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, or a seizure. Get seen in person 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.

Fever in children

Children's fevers are judged differently depending on age. We do not see babies under 6 months, and a baby under 3 months with a fever needs the emergency room. Read our page on fever in children.

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Frequently asked questions

What temperature is a fever in an adult?
Generally a temperature by mouth above 99 °F to 99.5 °F (37.2 °C to 37.5 °C), depending on the time of day. How you feel and whether you have warning signs matter as much as the number.
How many days of fever is too long for an adult?
Get checked if a fever lasts more than 48 to 72 hours, or keeps coming back for a week or more. If it stays at 103 °F (39.4 °C) or higher, do not wait that long.
What should I take for a fever if it might be dengue?
The CDC advises acetaminophen for dengue fever and pain, and no aspirin or ibuprofen. Drink plenty of fluids and watch for warning signs, especially as the fever starts to fall.
Can a telehealth doctor treat my fever?
If appropriate, the doctor can prescribe medication after a video assessment. We do not prescribe controlled substances. Many viral fevers only need home care, and antibiotics do not work on viruses. The doctor will explain what your situation needs, including when you should be seen in person.

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