“Reassortment” means the accidental swapping of entire genomic segments between virions of two different subtypes. It’s similar to recombination, as occurs sometimes between crossed chromosomes in dividing cells, except that reassortment is somewhat more facile and orderly. It happens often among influenza viruses because the segmentation allows their RNA to snap apart neatly at the points of demarcation between genes: those eight railroad cars in a switching yard.)