Vampire

Vampirism is a disease communicable by a magical, blood-based virus (sanguinavirus A) originally found only in the infernal bat (Desmodus infernus), a species that came from Averna, the nether dimension commonly known as Hell, which came to Earth in approximately 9000 BC. Observing that some of these bats were very hardy and resistant to damage, in the vicinity of what is now known as Northern Iraq, some sorcerers at the time chose to examine them in detail, and realised they were infected with the aforementioned disease. By artificially cultivating the virus for human transmission, they created a strain intended to infect mundane and magical humans (sanguinavirus B), and thus created the vampire.  
Today, according to records from the International Vampiric Council, there are approximately thirty to forty thousand vampires spread across the Earth, with the vast majority of them originally having mundane or magical humans that were infected with it. 
Sanguinavirus B is an extremely hardy virus that immediately makes changes to its host's make-up in order to ensure it can safely exist within it and sustain itself, and thus triggers what is broadly known as the vampiric disease.

What is a vampire?

The term "vampire" commonly refers to a human - ordinarily but not always from a magical community - who has been infected with Sanguinavirus B, and has subsequently contracted the disease known as vampirism. This is not to say that only humans can be vampires, however - many animals are capable of contracting vampirism, as are many fae species.
Once it has been transmitted to a new host, the sanguinavirus establishes itself in the core of its host's body, around the heart, and forms new matter that has been compared by some academics to a hindbrain. It devotes itself to initially changing the make-up of its host body, and then focuses itself on defending the host from threats, boosting immune response, strengthening the host, and ensuring it retains a temperature low enough for the virus to remain healthy. The vampiric core allows vampires to heal from even severe injuries very quickly, and makes them heavily resistant to damage or weakness.
As well as priming its host to resist damage or attack, the sanguinavirus creates in them a heavy thirst for blood, specifically for the iron-rich protein haemoglobin, and the vampire will be driven to quench this thirst in the same way they would be hunger or thirst for water.
A human vampire will typically develop an ability to extend two of their canine teeth, which will become very sharp, in order to pierce a victim's skin, and after making this piercing, they can lap at or suck from the blood that comes from the wounds. These teeth inject a vampiric venom, which is made up primarily of two parts - an anti-coagulant, to ensure free blood flow while the vampire drinks, and a strong analgesic that will impart a feeling of euphoria upon the victim. As a vampire ages, their venom typically becomes more potent, and stronger venom's effects can last for several days, and even have hallucinogenic effects, or induce memory loss. 
One of the primary changes in the vampire's body is in the increased density and number of tightly corded fiber that makes up their muscle: muscle becomes very tightly packed and heavy in make-up, meaning that many vampires are far stronger than their undiseased counterparts. The skin is also thickened and hardened, and the veins are tightly constricted in order to help keep them safe from harm and to ensure they are appropriately insulated by the heavy flesh. As a result of this strenghtening of both skin and muscle, a vampire is typically far, far heavier than their human counterpart, and most non-vampires would struggle to lift or support the weight of one.
With the thickening of the vampiric skin, the vampire ceases to sweat, and in such cases as the vampire becomes overheated, the veins will further constrict and the skin will become slightly thinner in an attempt to ensure heat can escape. In the case of the latter, the vampire's skin will seem chalky and strangely static in appearance, and these are the elements a doctor should look for as signs of fever, as opposed to a sweaty brow.
 Vampires do still need to eat food and remain hydrated, on top of their appetite for blood, and studies have shown that vampiric appetite is not noticeably different to that which they had whilst human, although some particularly strong-tasting or aromatic foods can become overpowering and thus less palatable once the disease has been contracted, owing to the enhancement of vampiric senses. With that said, vampires can survive starvation or extreme thirst for far longer than their human counterparts, even if they are also deprived of blood. 
The sanguinavirus thrives best at lower temperatures, and the body temperature of a human vampire typically stands at approximately 23° Celsius, or approximately 73° Fahrenheit. Vampires are subsequently very cold to the touch, as well as their flesh being hard when pressed upon, and their bodies are often compared to corpses in physicality. Vampires prefer cooler temperatures, and although due to the magical nature of their disease, they can often retain their homeostasis up to much higher temperatures, if a vampire contracts a fever, or if they are under threat from fire, it can rapidly become very dangerous for them.
Many vampires - although not all - are highly sensitive to direct sunlight, and have to shield themselves from the sun. It's generally recommended for vampires to shade themselves as best they can, wearing clothes that cover the skin, carrying a parasol or remaining in the shade, and to use a high-factor sunscreen. Those vampires sensitive to sunlight will often begin to burn and blister within minutes of being under direct sunshine, and there are documented cases - although none in recent history - of vampires dying from combustion following exposure to sunlight alone.
Many of the vampire's senses are heavily enhanced following the contraction of the disease, particularly their senses of hearing, taste, and smell. Due to the enhancement of the latter, some vampires find themselves sensitive to some strong scents - such as garlic, fish, or some strong chemicals - and vampires typically have strong enough hearing to keep track of the heartbeats of anyone else in their vicinity. 
Although the vampire's sense of sight is not particularly improved by the contraction of vampirism (it has been noted that both colourblind vampires and vampires with myopia or hyperopia, as well as vampires who are blind or partially sighted, retain their impediments post-turning), many vampires find that the extent to which their eyes take in light is heavily improved, meaning that they can see in low-light conditions, as well as being able to navigate by ear in complete darkness. As a result of the former, vampires can develop photosensitivity, particularly to the shine of bright sunlight or to strong fluorescent lighting, and will wear sunglasses in public. Vampires' homes are often lit by candles or by low-impact, warm lighting that is easy on the eyes.
For ancient vampires particularly, although these are problems many vampires have, adjusting to the modern world in the aftermath of the Industrial Revolution has been difficult because of the constant hum of electricity in wires, water moving through pipes, and particularly the high-frequency noises of motors and light bulbs. Due to the aggressive nature of the immune system after the vampiric disease has been contracted, some vampires also develop overpowering allergies, particularly to plastics, cleaning chemicals, and diesel and petroleum-based products. In urban and industrial areas, it is common to see vampires wearing clothes that cover them from head to toe, and to wear sunglasses or even goggles, and to wear some form of face covering, such as a mask or piece of gear to breathe through, and perhaps ear plugs.
Many vampires have highly developed sound proofing in their homes, or choose to live away from urban and industrial centres in order to avoid overstimulation.
Vampires are widely regarded as immortal, due to the fact that the effect of their disease is to ensure that they do not appear to age, and vampires can live for centuries upon centuries, if not millennia. This is not to say that the vampire is immune to damage - common causes of vampiric death are exposure to fire (although unlike humans, they do not die from smoke inhalation, but from the heat), hyperthermia, and as a result of fever from another contracted illness. 
Because of the effects of the vampiric "hindbrain", which encourages ultrafast healing in the vampire, and because the vampire's flesh is so resistant to being pierced or cut, it is difficult for a vampire to be injured. They can recover swiftly even after breaking a bone or severely injuring an organ, although they do not regenerate limbs. In cases of severe head injury, the "hindbrain" has been known to suddenly swell and take over many unconscious processes ordinarily controlled by the prime brain, such as control of the nervous, circulatory, and respiratory systems, giving the brain much needed time to heal itself. 
With that said, vampires are not capable of limb regeneration, and they cannot regenerate lost limbs, nor regrow their head once they have been beheaded. A traditional method of execution for a vampire is to drive a hard stake made of iron or heavy wood through their heart while they are lying on their back - this is because a hammer is needed to work up the necessary force to pierce the vampire's hard, dense chest, and the stake must pierce directly through the "hindbrain" to prevent it from assisting the vampire in later healing. 
Sanguinavirus can be transmitted between parent and baby, although there are some additional health risks concerned for expecting parents - some vampires struggle to drink enough blood whilst pregnant, as all the protein is consumed by the foetus, occasionally necessitating a regular transfusion. Contrary to popular belief, children who've contracted vampiric disease do continue to mature and grow, and although they have the teeth and a mild venom from birth, they do not form the strong, dense flesh, or sensitivity to light, that is typical to vampires until they reach puberty.
Young vampires tend to grow more consistently throughout puberty rather than having the sudden growth spurts human children often have, and as a result, many of them have complaints of sustained aches and pains until they're fully grown. With that said, on average, vampiric youths often grow taller on average than their human counterparts. They usually cease to visibly age at around the age of twenty-five. 
Being an artificially cultivated magical virus, sanguinavirus takes two elements to be transmitted - first, the new host must be bitten by a vampire, or to somehow have an infected vampire's venom in their veins, and secondly, they must then ingest the vampire's blood, or have it enter their bloodstream. 
A common myth among noble vampires, which has been roundly debunked by academics, is that the working class and poor vampires came about because they would capture and eat infernal bats that had drunk their blood. In actuality, there are vampires of all socioeconomic classes, because even at the advent of the vampiric disease's introduction to humanity, there were different sorcerers at work upon it, and later, noble vampires would often transmit the disease to their favourite servants or slaves, or to merchants and tradesmen in their communities.
It has been posited by many academics that there is a risk of sexual transmission of the sanguinavirus between a vampire and a partner from whom they have fed, because there is a potential for blood-to-blood contact because of tiny abrasions during sex. It is recommended that vampires use appropriate prophylactics during sex, even if they are using a chemical birth control, and to use plenty of lubricant. Full guidelines for safe sex for vampires can be found on the website of the International Vampiric Council.