dOocysts of the waterborne protozoan parasite Cryptosporidium parvum are highly resistant to chlorine disinfection. We show here that both silver nanoparticles (AgNPs) and silver ions significantly decrease oocyst viability, in a dose-dependent manner, between concentrations of 0.005 and 500 g/ml, as assessed by an excystation assay and the shell/sporozoite ratio. For percent excystation, the results are statistically significant for 500 g/ml of AgNPs, with reductions from 83% for the control to 33% with AgNPs. For Ag ions, the results were statistically significant at 500 and 5,000 g/ml, but the percent excystation values were reduced only to 66 and 62%, respectively, from 86% for the control. The sporozoite/shell ratio was affected to a greater extent following AgNP exposure, presumably because sporozoites are destroyed by interaction with NPs. We also demonstrated via hyperspectral imaging that there is a dual mode of interaction, with Ag ions entering the oocyst and destroying the sporozoites while AgNPs interact with the cell wall and, at high concentrations, are able to fully break the oocyst wall.