The present invention provides new compositions and methods for preventing and treating pathogen infection. In particular, the present invention provides compounds having an anchoring domain that anchors the compound to the surface of a target cell, and a therapeutic domain that can act extracellularly to prevent infection of a target cell by a pathogen, such as a virus. The present invention also comprises therapeutic compositions having sialidase activity, including protein-based compounds having sialidase catalytic domains. Compounds of the invention can be used for treating or preventing pathogen infection, and for treating and reducing allergic and inflammatory responses. The invention also provides compositions and methods for enhancing transduction of target cells by recombinant viruses. Such compositions and methods can be used in gene therapy.