Presently provided are methods for (a) modulating an activity of indoleamine 2,3-dioxygenase comprising contacting an indoleamine 2,3-dioxygenase with a modulation effective amount of a compound as described in one of the aspects described herein; (b) treating indoleamine 2,3-dioxygenase (IDO) mediated immunosuppression in a subject in need thereof, comprising administering an effective indoleamine 2,3-dioxygenase inhibiting amount of a compound as described in one of the aspects described herein; (c) treating a medical conditions that benefit from the inhibition of enzymatic activity of indoleamine-2,3-dioxygenase comprising administering an effective indoleamine 2,3-dioxygenase inhibiting amount of a compound as described in one of the aspects described herein; (d) enhancing the effectiveness of an anti-cancer treatment comprising administering an anti-cancer agent and a compound as described in one of the aspects described herein; (e) treating tumor-specific immunosuppression associated with cancer comprising administering an effective indoleamine 2,3-dioxygenase inhibiting amount of a compound as described in one of the aspects described herein; and (f) treating immunosuppression associated with an infectious disease, e.g., HIV-I infection, comprising administering an effective indoleamine 2,3-dioxygenase inhibiting amount a compound as described in one of the aspects described herein.