When shiitake mushrooms are infected with a virus and exposed to heat stress, the virus replicates more aggressively, which makes the mushrooms more susceptible to heat damage and competitive fungi. Researchers discovered that a small regulatory RNA molecule called led-milR-21 plays a key role in this process by suppressing the mushroom’s heat defense mechanisms when the virus is present. This discovery is important because it shows how viruses can exploit heat stress to overcome fungal defenses, with implications for mushroom cultivation in a warming climate.