R.J.
Yes the body runs fevers to fight infection HOWEVER (JUST like how baby snakes can't control how much venom they release, making them the most dangerous), babies bodies are NOTORIOUS with still figuring out the whole homeostasis thing / how strongly to react to infection. By the time kids are done teething (at about 2yo) their body usually has a handle on it. BUT even adults can spike fevers of 104+. You don't let someone die of a fever, you give them a fever reducer.
Fevers that high do NOT help, they hurt. It's our bodies freaking out / nervous system just flailing trying to stay alive. Like running away from a badguy into the street and getting hit by a car.
When someone has a history of febrile seizures TREAT AT FIRST SIGN. As they start getting better you can start stretching out the dose times to allow the body to learn how to self regulate. But don't play around in the beginning. Dose, and thank god we have the meds for it!!!
With fevers, it's "better low than high" safe/ than sorry. Her body will eventually learn how to use the "dimmer" switch on triggering fevers, but until then... use the tylenol/ motrin as training wheels.
Just a note: DO NOT NOT NOT put your baby or child in a cool bath. More than anything else, cool baths TRIGGER seizures in people of all ages with a fever. Their body goes into shock. Adults tend to have heart attacks and then seize, children tend to seize and then have heart attacks. LUKEWARM baths, otoh, are often recommended... but you reeeeally need to make sure that the water feels wrist temp or slightly warmer to YOU (will feel icy to her, but won't send her into shock). Hospitals put fever patients not responding to meds fast enough into either cool baths or ice baths all the time... but they have the crash cart right there with them, because on average about half the patients need to get shocked back to a normal rhythm. Even cooling blankets can trigger shock. It's just a very scary thing to be experimenting with at home.
So easy, peasy: dose at first sign. If it's still climbing after 15 minutes, add the other "type" of fever reducer (aka if you start with motrin, add tylenol.. if you start with tylenol, add motrin). As her fever lowers over the next few doses, subtract one and just stay on one type, then as it lowers further subtract the other.
BUT : As with all medical things, if you have questions, doubts, etc. PLEASE feel free to call your doctor or the 24/7 nurse hotline for honest to god medical advice. As long as you have a phone, you're never alone.