"To develop a fever" means that your body temperature is rising above normal and you feel hot. It usually happens when you are sick, as your immune system is fighting an illness or infection. Full definition
You don't have to refrigerate them, and they are portable so you can use them anytime or anywhere your child develops a fever. (earlymomentsmatter.org)
If your dog develops a fever, call the vet immediately. (straightpoop.com)
However, the baby may not develop a fever at all. (modernmom.com)