Force-separation measurements between a deformable, rough, biological surface (Cryptosporidium parvum) and an inorganic oxide (silica) have been obtained using the atomic force microscope. The system was chosen because oocysts of C. parvum have been associated with waterborne outbreaks of disease, and one of the main barriers to oocyst contamination of drinking waters is provided by sand-bed filtration. The oocysts are shown to be significantly rough on the scale of Derjaguin-Landau-Verwey-Overbeek forces and have been found to be compressible on the scale of the loads applied during force measurement. The surface compressibility is reported in terms of an interfacial spring constant. The force of interaction prior to this Hookean region is long-range and repulsive. The long-range force has been compared to models of the electrical double layer force (based on the measured zeta -potentials and bulk electrolyte concentration) as well as an electrosteric force (treating the surface as a polyelectrolyte brush). The comparison has led to the conclusion that the surface can be described as a polyelectrolyte brush at intermediate separations (approximate to 10-30 nm from linear compliance) with an electrical double layer often observed at greater separations. The dependence of the force on surface separation suggests that tethering between the oocyst and silica can occur. The variation of the interaction with pH and upon subtle changes in the ionic strength, compared to the variation from oocyst to oocyst, has also been assessed.