Origin:

According to Aleksander Brückner

Aleksander Brückner

Aleksander Brückner was a Polish scholar of Slavic languages and literatures , philologist, lexicographer and historian of literature. He is among the most notable Slavicists of the late 19th and early 20th centuries, and the first to prepare complete monographs on the history of Polish language...

, the word is derived from Strix

Strix

Strix may refer to:* Strix , a legendary creature of ancient Roman mythology* Strix , the genus of large "earless" wood-owls* Strix , a Swedish production company...

. Compare with Strigoi

Strigoi

In Romanian mythology, strigoi are the troubled souls of the dead rising from the grave. Some strigoi can be living people with certain magical properties. Some of the properties of the strigoi include: the ability to transform into an animal, invisibility, and the propensity to drain the vitality...

 a female demon with bird's claws who feeds on human blood. Unclear how strzyga was adapted by Slavic peaople probably through Balkan peaoples though it particulary took a form of a ghost.

Beliefs:

A strzyga (ˈstʂɨɡa) is a female demon a bit similar to vampire

Vampire

Vampires are mythological or folkloric beings who subsist by feeding on the life essence of living creatures, regardless of whether they are undead or a living person...

 in Slavic

Slavic peoples

The Slavic people are an Indo-European panethnicity living in Eastern Europe, Southeast Europe, North Asia and Central Asia. The term Slavic represents a broad ethno-linguistic group of people, who speak languages belonging to the Slavic language family and share, to varying degrees, certain...

 (and especially Polish

Poland

Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...

) folklore. Peaople who were born with two hearts and two souls and two sets of teeth(second one barely visible) were believed to be strzygas. Futhermore a newborn child with already developed teeth was also believed to surely become one. When strzyga was recognized it was chased away from human habitat. Strzygas usually were dying at young age, but only one soul gets passed on, and the other soul caused the deceased strzyga to come alive and prey upon other living beings. Those undead creatures fly at night in a form of an owl and attack night-time travellers and people who simply wander off into the woods at night, sucking out their blood and eating out their insides. Strzyga could also be satisfied with animal blood, for a short period of time.
When person recognized as strzyga dies Decapitating

Decapitation

Decapitation is the separation of the head from the body. Beheading typically refers to the act of intentional decapitation, e.g., as a means of murder or execution; it may be accomplished, for example, with an axe, sword, knife, wire, or by other more sophisticated means such as a guillotine...

 the corpse and burying the head separate from the rest of the body is said to prevent strzyga from rising back from the dead; burying the body face down with a sickle

Sickle

A sickle is a hand-held agricultural tool with a variously curved blade typically used for harvesting grain crops or cutting succulent forage chiefly for feeding livestock . Sickles have also been used as weapons, either in their original form or in various derivations.The diversity of sickles that...

 around its head is said to work as well.

According to the other sources strzygas don't do any other harm to peaople than heralding the certain death of one of family members(or one of inmates). At this point they are bit similar to Banshee

Banshee

The banshee , from the Irish bean sí is a feminine spirit in Irish mythology, usually seen as an omen of death and a messenger from the Otherworld....

.

In the times of epidemic sick peaople were burried alive. Some of them made their way out of grave with bare hands. Wandering around forests, weak and with bloody hands and torn-off fingernails. Those peaople were believed to be strzygas as well.