If you arrive at the entrance point with an undiagnosed fever, you may be denied access. Most of the time, you are not, but it depends on where you are coming from, how high your temperature is, if you have other symptoms of disease, and whether or not you are currently under the care of a doctor.
In most circumstances, your word is sufficient, but having proof to prove the condition is always preferable.
Traveling with medical equipment, medicines, or being disabled does not preclude you from visiting the United States; nonetheless, you must verify that you have the necessary documentation.
It may appear to be a lot of stress to deal with, but bear in mind that we do not live in a very secure environment, and taking these precautions keeps all travelers safe!
Just remember to document your ailment, condition, and/or prescription needs with your doctor on his or her letterhead, and you’ll be OK!