Vampires and Erythropoietic Protoporphyria patients
Firstly
Erythropoietic Protoporphyria (EPP) is a disease where pateints experience severs pain, when exposed to sunlight.
It's very similar to how vampires experience pain in sunlight.
Also since Erythropoietic Protoporphyria (EPP) patients avoid sunlight they have pale skin
Which also is character associated with vampired.
Patients with Erythropoietic Protoporphyria (EPP) can develop anemia where is needs blood.
This could be linked to vampire needing to drink the blood.
In past drinking blood directly was common as there was lack of transfusion techniques.. Historically, consuming blood was believed to restore vitality, increase strength, and cure illnesses related to paleness or fatigue(which Erythropoietic Protoporphyria patients suffer from).
There is another diseased Congenital Erythropoietic Porphyria(CEP) where patients too experience pain on exposure to sunlight.
Congenital Erythropoietic Porphyria (CEP), often called **Gunther disease**, is a rare genetic disorder caused by enzyme deficiency. This defect leads to a harmful buildup of porphyrins in the body, which causes a striking, characteristic triad of symptoms: severe skin photosensitivity, hemolytic anemia, and reddish-brown teeth(blood resemblance) (another character associated with vampires(bloody fangs/teeth)
I do think patients with Erythropoietic Protoporphyria EPP & CEP could be reason behind the Vampire myths.