Premise: Nosopsyllus fasciatus, the northern rat flea, is a species of flea found on domestic rats and house mice. Northern rat fleas are external parasites, living by hematophagy off the blood of rodents. It is the most widely spread of its genus, having originated in Europe, but has been transported to temperate regions all over the world.
Hypothesis: The northern rat flea enjoys living in parts of the planet where the climate is mild.