Methods and compositions for treating an infection or disease that results from (1) failure to elicit rapid T cell mediated responses, (2) induction of T cell exhaustion, T cell anergy or both, or (3) failure to activate monocytes, macrophages, dendritic cells and/or other APCs, for example, as required to kill intracellular pathogens. The method and compositions solve the problem of undesired T cell inhibition by binding to and blocking PD-I to prevent or reduce inhibitory signal transduction, or by binding to Hgands of PD-I such as PD-L1, thereby preventing (in whole or in part) the ligand from binding to PD-I to deliver an inhibitory signal. The immune response can be modulated by providing antagonists which bind with different affinity (i.e., more or less as required), by varying the dosage of agent which is administered, by intermittent dosing over a regime, and combinations thereof, that provides for dissociation of agent from the molecule to which it is bound prior to being administered again (similar to what occurs with antigen elicitation using priming and boosting). In some cases it may be particularly desirable to stimulate the immune system, then remove the stimulation.