Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
febrĭo, īre, v. n. [febris], to be ill of a fever, to have a fever (post-Aug.): si non febrit, venter solvendus est, Cels. 4, 4, 2; Col. 6, 38, 1; Macr. S. 7, 4.