Adults: call the doctor at 103 F, at 3 days of any fever, or sooner with severe headache, stiff neck, confusion, chest pain, breathing trouble, vomiting, or rash. ER: 105 F, or fever with confusion, seizure, stiff neck, or a non-blanching purple rash.
Context matters: fever is defense, most infection fevers stay under 105 F, and behavior beats the number; a coherent 103 F adult is safer than a confused 101 F one. Kids differ: 100.4 F under 3 months old is always a same-day call. Lower thresholds apply to the immunocompromised, pregnant women, recent surgery patients, and the elderly, who can be dangerously ill with confusion and little fever. Home care while monitoring: aggressive fluids, light clothing, acetaminophen or ibuprofen for comfort, and temperature checks every few hours, not every few minutes. Recurring fevers over weeks are a workup, not a wait-and-see.
What to do
Common questions
When is a fever too high?
For adults, the MedlinePlus thresholds: call the doctor at 103 F (39.4 C) or higher, at any fever lasting past 3 days, or sooner if fever comes with severe headache, stiff neck, confusion, chest pain, trouble breathing, persistent vomiting, a rash, or pain when urinating. The ER line: 105 F (40.6 C) or higher, or fever with confusion, seizure, stiff neck, severe breathing difficulty, or a non-blanching purple rash. The useful context: fever is a defense mechanism, most infection-driven fevers stay under 105 F, and the number alone is less important than how you look and act; a coherent 103 F adult is safer than a confused 101 F one. Children run different math: any fever of 100.4 F (38 C) in a baby under 3 months is a same-day medical call, and kids with fever plus lethargy, breathing trouble, stiff neck, or non-blanching rash go to the ER. Special adult populations with lower thresholds: the immunocompromised (chemo, transplant, high-dose steroids), pregnant women, recent surgery patients, and the elderly, who can be seriously infected with little or no fever at all, so confusion without fever counts too. Home management while watching: fluids aggressively, light clothing, acetaminophen or ibuprofen for comfort (they treat misery, not the cause), and recheck the temperature every few hours rather than every few minutes. The pattern that always earns a workup: fever that keeps returning over weeks, which is a "fever of unknown origin" workup, not a wait-it-out situation.