Up Until Ca. 1900
In slightly newer tradition, the gjenganger remains a violent entity, though in a less direct way, now becoming more of a disease-spreader. These gjengangere would attack people with their so-called dødningeknip (dead man's pinch). This would result in the living persons skin becoming sunken and blue where the gjenganger had pinched them, and this often led to disease and death for the afflicted person. The pinch was often administered when the person was asleep. Both the huldrefolk and nøkken were also accused of doing the same, using bites instead of pinches, often aimed at the victims face. This belief in beings attacking people in their sleep was used as a warning against going to sleep in specific places (near the graveyard, mountains or water respectively).
In later Swedish folklore, a distinction is made between the traditional gjenganger, in Swedish called gengångare, and another type of ghost known as gast. Whereas the gengångare looked virtually identical to a living human, the gast was known to be transparent and/or skeletal in appearance, sometimes it also had sharp fangs and claws, thus making it impossible to see who the phantom had been while alive. And whereas the Swedish version of the gengångare (unlike its counterparts in other Scandinavian countries) were usually said to be rather harmless, it was the gast who was known to cause diseases. They were also known to cause accidents and scare people for no apparent reason other than that they enjoyed doing so.
Read more about this topic: Gjenganger