Amyloid fibers formed from a peptide ubiquitous in human seminal fluid (SEVI) have been found to dramatically enhance the infectivity of the HIV virus (3-5 orders of magnitude by some measures). To complement these previous in vivo studies we have performed in vitro assays of PAP248-286, the most active precursor to SEVI, and other polycationic polymers to investigate the physical mechanisms by which the PAP248-286 promotes the interaction with lipid bilayers. At acidic, but not at neutral, pH freshly dissolved PAP248-286 catalyzes the formation of large lipid flocculates in a variety of membrane compositions which may be linked to the promotion of convective transport in the vaginal environment rather transport by a random Brownian motion. Furthermore, PAP248-286 is itself fusiogenic and weakens the integrity of the membrane in such a way that may promote fusion by the HIV gp41 protein. A partially α-helical conformation of PAP248-286, lying parallel to the membrane surface, is implicated in promoting bridging interactions between membranes by the screening of the electrostatic repulsion that occurs when two membranes are brought into close contact. This suggests non-specific binding of monomeric or small non-fibrillar oligomeric forms of SEVI to lipid membranes may be an additional mechanism by which SEVI enhances the infectivity of the HIV virus. In addition, we show that the curli protein, a strongly amyloidogenic protein used by bacteria to adhere to surfaces, seeds SEVI amyloid formation, indicating a possible role for bacterial infection in HIV transmission.