The most widespread evidence for parasite infection in medieval Europe is for species spread by poor sanitation, such as whipworm, roundworm, and the protozoa Entamoeba histolytica and Giardia duodenalis, likely related to the common use of human faeces as a crop fertilizer. The prevalence of infection by intestinal helminths has been shown to be at least a quarter to a third of the medieval population, with a broad north–south divide in the dominant types of parasite. While species spread by poor sanitation were present throughout the continent, in northern Europe where eating raw, smoked, dried, or pickled fish was common, fish tapeworm resulted. The use of dogs by farmers put them at risk of infection by Echinococcus granulosus as shown by calcified hydatid cysts. Human fleas and body lice helped spread the Black Death pandemic from the fourteenth century onwards. Medical practitioners thought intestinal worms were formed due to an excess of phlegm (one of the four bodily humours), while ectoparasites were formed due to putrefying humours, sweat, and grime. Delousing combs were widely used to remove head lice, while the wealthy in Italy applied mercury ointments to their hair.