Genetic chimerism is rarely considered in the analysis of population genetics data, because assumed to be an exceptionally rare, mostly benign, developmental accident. An unappreciated source of chimerism is transmissible cancer, when malignant cells have become independent parasites and can infect other individuals. Parasitic cancers were thought to be rare exceptions, only reported in dogs (Murgia etal., Cell, 2006, 126, 477; Rebbeck etal., Evolution, 2009, 63, 2340), Tasmanian devils (Pearse and Swift, Nature, 2006, 439, 549; Pye etal., Proceedings of the National Academy of Sciences, 2016, 113, 374), and soft-shell clams (Metzger etal., Cell, 2015, 161, 255). However, the recent simultaneous report of four new contagious leukemias in marine mollusks (Metzger etal., Nature, 2016, 534, 705) might change the rules. By doubling up the number of naturally occurring transmissible cancers, this discovery suggests they may essentially be missed because not sufficiently searched for, especially outside mammals. We encourage population geneticists to keep in mind infectious cancer when interpreting weird genotypes in their molecular data. It would then contribute in the investigation of how widespread contagious cancer could really be in the wild. We provide an example with our own data in Mytilus mussels, a commercially important shellfish. We identified genetic chimerism in a few mussels that suggests the possible occurrence at low prevalence in European M.edulis populations of a M.trossulus contagious cancer related to the one described by Metzger etal. (Nature, 2016, 534, 705) in populations of British Columbia.