You should not go to work with a fever. The standard guidance from the CDC is to stay home until your fever has been gone for at least 24 hours without using fever-reducing medication like ibuprofen or acetaminophen. A fever generally means an oral temperature above 100.0°F (37.8°C), though readings vary slightly depending on the time of day and how you measure.
What Counts as a Fever
Normal body temperature isn’t a single fixed number. It fluctuates throughout the day, running lower in the morning and higher in the late afternoon. A morning oral temperature above 99°F (37.2°C) or a late afternoon oral reading above 99.9°F (37.7°C) is generally considered elevated. Most clinical references define a clear fever as an oral temperature above 100.0°F or a rectal temperature above 100.8°F.
If you’re using a forehead thermometer, be aware that these are the least accurate consumer option. Ear (tympanic) thermometers are significantly more reliable, with studies showing around 83% sensitivity for detecting true fevers compared to core body temperature. Oral thermometers fall somewhere in between but can be thrown off by recent hot or cold drinks. If your forehead thermometer shows a borderline reading, try confirming with an ear or oral thermometer before assuming you’re in the clear.
Why the 24-Hour Rule Exists
The CDC’s updated respiratory virus guidance, released in 2024, simplified recommendations across flu, COVID-19, and RSV into one consistent message: you can return to normal activities when your symptoms are improving overall and any fever has been gone for at least 24 hours without medication. That last part is critical. Taking ibuprofen or acetaminophen and seeing your temperature drop doesn’t mean your fever has resolved. It means the medication is temporarily suppressing it.
Research published in Proceedings of the Royal Society B found that suppressing fever with medication may actually increase how much virus you shed and how long you shed it. The study estimated that widespread fever suppression enhances influenza transmission by at least 1% at the population level. That might sound small, but it translates to thousands of additional infections during a typical flu season. On an individual level, the concern is straightforward: you feel well enough to go in, but you’re still contagious and now interacting closely with coworkers.
How Long You’re Contagious
With most respiratory viruses, you’re most contagious in the first few days after symptoms appear. For COVID-19, viral shedding peaks within the first five days of symptoms. The percentage of people still carrying viable, transmissible virus drops steeply after that: roughly 44% to 50% in the first five days, falling to 28% by day seven, 11% by day nine, and essentially zero by day 18. The flu follows a similar early-peak pattern, with most adults contagious from about one day before symptoms start through five to seven days after.
A fever is your body’s signal that your immune system is actively fighting an infection, which typically overlaps with the period of highest contagiousness. Once the fever breaks on its own, it’s a reasonable indicator that the worst of the infectious period is winding down, which is part of why the 24-hour fever-free rule works as a practical guideline.
Fever and Your Ability to Work Safely
Even if you’re not worried about spreading illness, fever impairs your ability to function. Elevated core body temperature is associated with slower reaction times, reduced accuracy on tasks requiring focus, and impaired short-term memory. Research from the CDC examining the effects of elevated body temperature on cognitive performance found measurable declines in reaction time and accuracy on attention-based tasks. While that study focused on heat-related strain rather than infection, the underlying mechanism is the same: a hotter brain doesn’t perform as well.
This matters most in jobs involving driving, operating machinery, making safety-critical decisions, or caring for others. But even at a desk job, you’re likely to produce lower-quality work, make more errors, and recover more slowly than if you’d stayed home. Pushing through a fever often extends the total duration of illness, meaning more days of reduced performance overall.
What to Do Before Going Back
Check your temperature at least twice, once in the morning and once in the afternoon, on the day you think your fever has broken. Use an ear or oral thermometer for the most reliable reading. If both readings are below 100.0°F and you haven’t taken any fever-reducing medication in the past 24 hours, that’s a good sign. Wait until a full 24-hour period passes with normal readings before heading in.
When you do return, your other symptoms should also be trending in the right direction. A lingering cough or mild fatigue is normal during recovery, but if you still have body aches, chills, or worsening congestion, your body is telling you it’s not finished fighting. For the first few days back, wearing a mask in shared spaces further reduces the chance of passing anything along, since low-level viral shedding can continue even after fever resolves.
When a Fever Needs Medical Attention
Most fevers from common infections resolve on their own within a few days. But certain symptoms alongside a fever point to something more serious. Seek medical care promptly if you experience confusion, a stiff neck, seizures, trouble breathing, severe pain, or loss of consciousness. These can indicate infections like meningitis or complications that need immediate treatment. A fever above 103°F (39.5°C) that doesn’t respond to medication also warrants a call to your doctor, as does any fever lasting more than three days without improvement.