Shtriga

The Albanian: Shtriga (Latin:strix, Italian: strega, Romanian: striga, Polish: strzyga), in Albanian folklore, was a vampiric witch that would suck the blood of infants at night while they slept, and would then turn into a flying insect (traditionally a moth,fly or bee). Only the shtriga herself could cure those she had drained (often by spitting in their mouths), and those who were not cured inevitably sickened and died. Also means just a witch, a person that uses magic. The male noun for shtriga is shtrigu or shtrigan.

The television show Supernatural had an episode on a Shtriga who Dean believes attacked Sam during their childhood. The Shtriga could only be killed by an iron round while feeding, and it fed on "spiritus vitae" "breath of life". The shtriga was revealed to be a male pediatrician at the town hospital.

TV SHOW SAYS

It's kind of a witch. They're Albanian but the legends about them dates back to Ancient Rome. They need of spiritus vitae(breath of life). Kind of like your life force or essence. She/he takes your vitae your immunity goes to hell, pneumonia takes hold.

Sthrigas can feed of anyone but they prefer children. Probably becouse they have strong life force.

The kids aren't responding to antibiotics. Their white cell count keeps going down. Their immune systems just aren't doing their job. It's like their bodies wearing out.

The way it spreads, it works its way through families but only the children; one sibling after another...

Sthrigas are invulnerable to all weapons devised by God and man. She's only vulnerable when she feeds. If you cath her while eating, you can blast her with consecrated wrought iron. Buckshots or rounds.

Sthrigas take on human disguise when they're not hunting. historically, someting innocuous. Could be anything but it's usually a feeble old women which may be how the whole witches-as-old-crones legend got started.