The present invention relates to methods and compositions for preventing or treating diseases of a mammal, including viral infections, wherein at least one symptom of the disease is mediated at least in part by the binding of an effector molecule to a DC-SIGN receptor present on cells of the mammal to be treated. The invention also provides methods of identifying compositions, wherein the compositions are useful for treating mammalian diseases, including viral infections, for which at least one symptom of the disease is mediated at least in part by the specific binding of an effector molecule to a DC-SIGN receptor present on the cells that express the DC-SIGN receptor, belonging to the mammal to be treated. The invention further relates to compositions and methods for targeting subject molecules to cells that express the DC-SIGN receptor.