Do parasites weaken their hosts' resilience to environmental stress? Not always, according to this study. Rather than weakening its brine shrimp intermediate host, tapeworm infection enhances the shrimps' ability to cope with arsenic contamination in the water, and the same holds true in the warmer waters predicted by climate change models. Brine shrimps were collected from a highly polluted estuary in Spain and their infection status with tapeworm larvae was determined. Samples were used for toxicity testing with arsenic. To their surprise, the researchers found that infected shrimp were consistently more resistant to arsenic than uninfected ones. This was true not only at 25 degrees Celsius (the temperature under which both samples were tested), but also at 29 degrees (tested on some of the shrimp from the larger May sample). Overall, the 4-degree increase—consistent with current climate-change predictions for the change in mean temperature—made the shrimp more vulnerable to arsenic toxicity. To examine how parasite infection might protect the shrimp against arsenic toxicity, the researchers collected another sample from the same location in May 2015. Infection details were similar to the May 2014 sample. Comparing infected and uninfected Artemia, they found increased numbers of fat-containing droplets in the infected shrimp. Parasite infection was also associated with significant changes in oxidative stress markers. Lipids such as those in lipid droplets are thought to be able to protect organisms against pollutants by sequestering toxins away from sensitive target sites—a principle known as 'survival of the fattest'. Regarding oxidative stress, the researchers speculate that the tapeworm parasites benefit from healthy intermediate hosts with high chances of becoming tasty food for flamingos, grebes and other final avian hosts. This study provides the first empirical evidence that parasites can increase resistance to metal or metalloid pollution, rather than decrease it. It is also the first study to consider the influence of temperature change on parasite-pollutant interactions. Results contradict the pre-existing view that pollution and parasites are stressors that both have negative effects on the health of free living organisms. informacion[at]ebd.csic.es: Sánchez et al (2016) When parasites are good for health: cestode parasitism increases resistance to arsenic in brine shrimps. PLoS Pathog 12(3): e1005459. doi:10.1371/journal.