Along the quiet border waters of Lake Memphremagog, where Quebec meets Vermont, something has been moving through the brown bullhead catfish that defies one of biology's oldest assumptions: that cancer is a solitary fate, born and buried within a single body. Scientists have now confirmed, in a study published in Nature, that melanoma tumours are transmitting directly between fish — only the handful of such contagious cancers ever documented in any animal. The discovery does not merely mark a biological anomaly; it opens a deeper question about what conditions in the living world allow disease