PS Most people think a fever is a higher than usual temperature. But the medical community generally doesn’t recognize someone as having a fever until the temperature is at or above 100.4 degrees Fahrenheit.

“We all have a core temperature and then a fever is an abnormal temperature,” he says Thomas Russo, MD, professor and chief of infectious diseases at the University at Buffalo in New York. He points out that as you age, your core temperature can drop. As a result, you may feel feverish at a temperature below 100.4 degrees.

First things first: Do you need a thermometer to know if you have a fever?

Technically, yes. “You can feel a fever without having a temperature, but clinically you can’t have a fever without a temperature over 100.4 degrees Fahrenheit on the mouth,” says Eric Ascher, DO, family physician at Northwell Lenox Hill Hospital.

That doesn’t mean you don’t have a fever if you don’t have a thermometer nearby—you just can’t tell for sure.

How can you tell if you have a fever without a thermometer?

If you suspect you have a fever but don’t have a thermometer around (and don’t feel like investing in one), doctors say there are a few signs of a fever to watch out for:

• You feel uncomfortable. “We usually don’t feel great with a fever,” says Dr. Russo. “You just feel uncomfortable.”
• You feel hot. You may feel like you’re overheated, even when others feel fine,

  Richard Ricciardidr., executive director of the Center for Health Policy and Media Engagement at George Washington University School of Nursing, says. You may also feel sweaty.

• You feel cold. On the other hand, having a fever can cause you to shiver or even start shaking, according to Dr. Ricciardi.
• Your forehead and cheeks are warm. Feeling these areas, or having someone else feel them for you, can indicate whether you have a fever, Dr. Ascher says.
• you are weak. When you have a fever, your body will often divert energy to trying to fight off an infection, says Dr. Russo. As a result, you may end up feeling tired or weak.
• You are not hungry. Having a fever can often destroy your appetite, says Dr. Ascher.

How long does a fever usually last?

How long your fever lasts will usually depend on what’s behind it, Dr. Russo says. “People who have a self-limited cause of fever, such as a viral infection you’re recovering from, may have a fever for a few days,” he says. “For some people, the fever may last for a longer period of time.”

Generally, Dr. Ascher says many will last two to three days.

How to treat a fever

There are a few different things you can do to treat a fever, according to Dr. Ricciardi:

• Rest
• Drink plenty of fluids to reduce the risk of dehydration
• Take fever-reducing medications such as ibuprofen and acetaminophen

“I tell my patients if they never get better within 24 to 48 hours, even with medication, to call their doctor,” says Dr. Ascher.

Can you have a fever without a temperature?

No, you can’t have a fever without a temperature. “Having a fever means you have an elevated body temperature,” says Dr. Ricciardi.

Dr. Russo points out, however, that there are different variations of fever. “There are low fevers and much higher fevers,” he says. “There isn’t a perfect correlation between the higher the fever and the more severe the illness, but a very high fever raises the concern that the illness may have a bacterial cause, other than the flu or COVID.”

When to go to the doctor for a fever

Dr. Ricciardi recommends seeing a doctor for a fever if you have any of these symptoms:

• Fever lasts more than 72 hours
• You feel very bad
• You have difficulty eating, drinking, walking or carrying out regular daily activities

“It’s also recommended to seek medical attention if you have a severe headache, stiff neck, difficulty breathing, chest pain, confusion, persistent vomiting, and a rash,” he says.

In general, Dr. Russo says it’s important to remember that a fever is an indication of an infection. “You should have a low threshold for seeing a doctor for a high fever,” he says.