Medical Definition of cofactor
1: a substance that acts with another substance to bring about certain effects; especially : coenzyme
2: something (as a diet or virus) that acts with or aids another factor in causing disease <Infection with the HPV virus is an important cofactor in cervical cancer, but very few people infected with HPV will ever get cervical cancer.—Elissa Mendenhall, Mothering, May/June 2007>
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