feeble, febrile

Feeble or feeble means weak or debilitated, literally or figuratively --
He is getting old and feeble; he needs help to get around.
He made a feeble protest, but went along in the end.

Febrile or febrile means feverish, related to or caused by fever, physically or mentally--
A temperature that high can cause a febrile seizure.
His suspicions are the product of his febrile imagination.

Now that you know that, you can say --
"He is still feeble after a week in the hospital with an unknown febrile illness."