Organisms that live in large social groups benefit from enhanced cooperation, but also face increased threats from pathogens and parasites. In particular, wood ants live in crowded, persistent, warm and resource-rich nests providing ideal conditions for the development and spread of micro-organisms. We observed that Formica paralugubris workers often incorporate large quantities of solidified conifer resin in their nests. In a series of experiments, we tested the hypothesis that the resin protects the ants against pathogens. By creating resin-free and resin-rich experimental nests, we demonstrated that the presence of resin decreased the density of bacteria and fungi in nest material (Christe P. 2003. Ecol. Lett. 6: 19-22). We then identified several micro-organisms that were inhibited by resin in vitro. We selected two of them to experimentally infect ants in presence and absence of resin. Both the gram-negative bacteria Pseudomonas fluorescens and fungus Metarhizium anisopliae caused significant mortality to adult workers and larvae. Most importantly, the presence of resin strongly increased the survival of larvae experimentally infected with both micro-organisms, as well as the survival of adults infected with P. fluorescens. Finally, in choice experiments we found an active preference for resin, but this preference was independent of experimental infection with M. anisopliae.   Altogether, these results show that the resin protects the ants from virulent fungal and bacterial infections. The collection of resin appears to be an active, constitutive behaviour, rather than a conditional response to infection, at least with respect to M. anisopliae. More generally, the collective use of plant products with medicinal properties might be an important, although not well documented, mean of defence against parasites in some animal societies.