Physiological / biochemical explanation

Dracula is usually described as a man with pointed ears, sharp teeth (with blood sometimes), and hair on his palms, who hides somewhere during the daylight and needs blood.

(Public domain, from Wikipedia)

Oxygen is carried in blood by heme which is a component of haemoglobin (Hb).
Heme is synthesized in eight steps (at each step a special enzyme is used) in the mitochondria and cytosol of the liver cells (hepatocytes) and developing red cells in the marrow. This process could be illustrated like this

(The image above was created by me and goes into Public domain)
(Substrates are denoted by gold stars (and orange rhombuses), enzymes by brown lightnings. At the end of the eighth step heme is produced by combining Protoporphyrin IX with Fe2+ ions. Then it is transported out of the mitochondrion to the cytosol where it’s combined with globin protein chains. Fourth step is catalyzed by Uroporphyrinogen-III Synthase (purple arrow))

What we are interested in is that fourth step catalyzed by the enzyme Uroporphyrinogen-III Synthase (or URO3S), where the substrate hydroxymethylbilane (or HMB) turns into Uroporphyrinogen III (or URO'GEN III; it’s highlighted with the help of blue oval) [1].

(The image above was created by me and goes into Public domain)

This happens in most of the human population, but in some rare cases (less than 1 in 1,000,000) different mutations of the URO3S gene (which is located at 10q25.2–26.3 chromosome) can happen [2]. This leads to a condition called congenital erythropoietic porphyria (CEP, or Gunthers disease). One study shows, however, that CEP can be also caused by GATA1 mutations (transcription factor) rather than URO3S mutations [3].

That word “Porphyria” originates from the root for “purple” in Greek and was given by German student Schultz in 1874.[[4]]

The abnormal enzyme is unable to catalyze HMB properly which leads to its spontaneous cyclization that results in Uroporphyrinogen I (URO'GEN I; look at the blue semicircle again) production. This URO'GEN I cannot be converted into heme properly.

(The image above was created by me and goes into Public domain)

As a result uroporphyrin I (URO I) accumulates in erythrocytes. This leads to skin photosensitivity, hence skin wounds in patients. However, the above situation doesn’t mean that the normal URO3S doesn’t exist in those patients at all. But that normal URO3S activity is reduced drastically in most CEP patients.[5]

Abnormal toxic substrates accumulation happens in different tissues (skin, teeth, liver, bone marrow), urine, and feces, and leads to a wide range of symptoms which include photosensitivity (porphyrins are fluorescent molecules; so those “vampires” need to hide from the sun and wear protective clothing), skin wounds, red urine, fluorescent teeth, thicker than normal skin, erythrodontia (teeth become reddish), disgust to garlic, wish to drink blood.[4]
Also peripheral phalanges of the fingers become shorter, and the skin of the fingers become tighter than normal (aka “werewolf hands”). [6]
And there might be loss of eyelashes and eyebrows, hypertrichosis (excess amount of hair) on the limbs, back, and face (aka ”werewolf syndrome”). [7]

Anemia (deficiency of proper haemoglobin or red blood cells) is characterized by weariness and pallor.
Some patients with CEP might experience most of the symptoms mentioned above while others only a few of the symptoms.

Psychological explanation

Now, when we discussed one possible explanation on the physiological level, let’s talk about those cases when people without CEP tries to mimic vampire/werewolf behaviour.

In 1961 Albert Bandura showed that after children were demonstrated adults’ behaviour (and those adults were beating an inflatable clown named “Bobo”), they began to mimic that behaviour and display more aggression compared with those children who weren’t demonstrated that kind of behaviour.
So, there appeared a suggestion that people can learn by mimicking others’ behaviour. [8]

Physiological basis for the results above could be mirror neurons. These neurons fire when animals perform some actions as well as when those animals see/hear that someone else do those same actions, so that they “mirror” others’ behaviour. This phenomenon can also serve as a basis for empathy. [9]

(Creative Commons Attribution 2.5 Generic, Evolution of Neonatal Imitation. Gross L, PLoS Biology Vol. 4/9/2006)

You can watch one short interesting TED talk on this topic.

There’s also lycanthropy – condition when people think that they are animals (usually wolves).

Possibly, those cases reported by mass media could be explained by the above findings.

"Trojan Horse", metaphorically speaking, refers to a trick which induce protected system to let enemies enter. So, maybe, we could refer to those mirror neurons as a "Trojan Horse" which helps the enemies (in the context of this post it's the behaviour observed in films or somewhere) to be perceived by protected system (in the context of this post it's a human brain).

Personal experience

If you have (or had) parents, possibly, you could notice that you act (and speak) sometimes in a very similar way to them.
The brightest example in my life is the little boy who lived on our street one day. I knew that his father used bad language a lot. And that little boy (3-5 years old) was the ultimate champion in that activity on our street.

When did you begin to place that banner “Upvote/Resteem…” at the bottom of your posts?
Maybe the finding mentioned above could also explain this behaviour?

PS: just imagine that the person with CEP is also a big fun of “Twilight” or something similar. What could we expect?
... )

References

1. Hamza I, Dailey HA. One ring to rule them all: trafficking of heme and heme synthesis intermediates in the metazoans. Biochim Biophys Acta. 2012, Sep;1823(9):1617-32. doi: 10.1016/j.bbamcr.2012.04.009. Epub 2012 May 8. Review. PubMed PMID: 22575458; PubMed Central PMCID: PMC3412874

2. Thadani H, Deacon A, Peters T. Diagnosis and management of porphyria. BMJ. 2000 Jun 17;320(7250):1647-51. Review. PubMed PMID: 10856069; PubMed Central PMCID: PMC1127427

3. Phillips JD, Steensma DP, Pulsipher MA, Spangrude GJ, Kushner JP. Congenital erythropoietic porphyria due to a mutation in GATA1: the first trans-acting mutation causative for a human porphyria. Blood. 2007 Mar 15;109(6):2618-21. Epub 2006 Dec 5. PubMed PMID: 17148589; PubMed Central PMCID: PMC1852202

4. Congenital Erythropoietic Porphyria – Gunther's. Disease Basis for Werewolves and Vampires. Syed Muarraf Hussain , Syed Irfan Ahmed* Journal of Rawalpindi Medical College (JRMC); 2013;17(2):300-301 (can be found on http://www.journalrmc.com/index.php/JRMC)

5. Ajioka RS, Phillips JD, Kushner JP. Biosynthesis of heme in mammals. Biochim Biophys Acta. 2006 Jul;1763(7):723-36. Epub 2006 Jun 3. Review. PubMed PMID: 16839620

6. Sharma S, Chauhan PS, Sharma V, Sharma N. Dark age vampires or our poor patients. Indian J Dermatol. 2013 Sep;58(5):412. doi: 10.4103/0019-5154.117375. PubMed PMID: 24082237; PubMed Central PMCID: PMC3778832

7. Koley S, Saoji V. Congenital erythropoietic porphyria: two case reports. Indian J Dermatol. 2011 Jan;56(1):94-7. doi: 10.4103/0019-5154.77565. PubMed, PMID: 21572804; PubMed Central PMCID: PMC3088948

8. https://www.psychologicalscience.org/publications/observer/obsonline/bandura-and-bobo.html

9. Keysers C. Mirror neurons. Volume 19, ISSUE 21, PR971-R973, November 17, 2009

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