Our 5-week-old has a temperature of 100.

Go to the pediatric emergency department now — leave within the next few minutes, tonight, not first thing in the morning. A 5-week-old with 100.6 °F (38.1 °C) is the exact situation that gets looked at in person the same hour, and if at any point he looks limp, hard to wake, is not feeding, or is breathing fast or grunting, call your local emergency number now instead of driving.

Why this age is different
Below about 3 months, a baby's immune system and circulation are still immature, and infections that stay mild in an older baby can spread quickly in a very young one. The clinical guidance behind this is blunt: a temperature of 38 °C or above in a baby this young warrants in-person investigation and treatment, and being "a bit warm" or "he looks fine to me" is not a reason to wait. The number is not the point — a well-looking 5-week-old still gets seen, and a high temperature is not what decides it. I am not diagnosing anything and I cannot tell you what is causing this; that is exactly the work the department does tonight.

1. What you tried vs. what helps instead

• Bundling him up to sweat it out — drop it. Extra layers trap heat and can push the temperature up. Light clothing, one more layer than a comfortable adult, a warm but ventilated room. You can still hold him skin to skin for warmth on the way out.
• Do not wait for the number to come down as your plan. Keep a written log anyway (time, how you took it, reading) — the staff will want it, and it is evidence, not a decision rule.
• Do feed him on his usual schedule on the way unless he refuses or the team tells you to stop; bring your usual feeding supplies. Do not force food or drink into a baby who is not fully awake.
• On the way (about 20 minutes, three jobs): dress him light, keep him held upright on your chest or in the carrier, pack the diaper bag with the thermometer, a spare set of clothes, feeding supplies, and any medicine or supplement he takes, plus your immunisation record if you have it. If someone is home, have them write down the last feeds and wet nappies so you are not trying to remember in the car.
• Call ahead if you can so triage knows you are coming with a febrile 5-week-old while you are still on the road.

2. Avoid list (home)

• No cold baths, no alcohol on the skin, no ice packs, no "teething fits" explanation for a fever at this age.
• No new medicine, no leftover antibiotics, no alternating fever medicines, no anything bought online, no powders or remedies given by someone else's child. Any comfort medicine is for comfort only, does not change the illness, and is a decision for the team or the label on the bottle — I am not giving you a dose.
• Do not give aspirin. Do not change or add anything he already takes.
• Do not read the thermometer as a stay-home rule, and do not take "the temperature came down" as good news on its own — that is not what decides this.

3. Decision tree — stop DIY and get clinical help

• Right now, as described: baby under 3 months with a measured fever → pediatric emergency department, now. If he is floppy, unresponsive, having a fit, struggling to breathe, grunting, sucking in under the ribs, going blue, has a rash that does not fade when you press a glass on it, has a bulging soft spot, is vomiting green, or is simply not feeding or much quieter than usual → call your local emergency number and follow the dispatcher.
• At the hospital: they will re-check his temperature, look at how alert and warm he is, ask about feeds and wet nappies, and very likely take blood for tests, and possibly a urine or throat sample. Many departments will keep a baby this young under observation for a few hours. Anything further — fluids, antibiotics, a lumbar puncture — is their call based on what they find, and I am deliberately not walking you through that: those decisions belong to the team in front of you, and the guidance I work from does not cover hospital procedures.
• After they discharge him: follow their written instructions exactly, including the exact "come back if" list and any review appointment. Do not treat a normal night at home as a reason to skip the review.
• If you are not sure whether to call an ambulance or drive: if he looks well enough to feed and be held awake on your chest, and the drive is 20 minutes with a second adult who can watch his breathing, driving is reasonable. If any of that is unclear, call.

What "going well" looks like in the next 24 hours
Not "the fever is gone." It looks like: he is seen tonight; you leave with a diagnosis or a clear plan, a written return list, and any follow-up appointment booked. Then at home, he feeds as usual and is making his usual wet nappies, he is awake and settled rather than limp or unusually sleepy, his breathing is quiet and easy, and any temperature change is read together with how he looks and behaves, not on its own. A fever that lingers is common and is not a failure — what matters is the feeding, the nappies, and the alertness. I cannot promise this resolves tonight; most of these visits end with a plan rather than a cure, and that is the normal outcome.

Script you can screenshot and send

• To your partner, parent, or the sibling/elder: "He's 5 weeks old and has a fever of 100.6. That's the age where they want him checked in person tonight, not tomorrow. I'm taking him to the pediatric ER now, it's a 20-minute drive. Can you [hold the other kids / come with me / get his feeding things together]? I'll text you from the department."
• To work or school, for tomorrow: "I have a family medical emergency tonight and may be off in the morning. I'll update you as soon as I can — please don't wait up for me."
• To the triage desk: "Five weeks old, rectal temperature 100.6 at [time], first noticed [time]. Last feed [time], last wet nappy [time]. He's [feeding as usual / fussier / harder to wake]. Immunisations: [as far as I know]. Nothing given except [what, if anything]."

One thing I still need, because it changes how fast you go
How is he right now — feeding and wet nappies as usual, and alert and settled when you pick him up, or is any of that slipping? And if you tell me your city or country, I can point you to the right local emergency number to have on hand on the road.