The vampires won't hurt you: but the bacteria might
We all know the vampire tropes – interpreted in so many different ways from Bram Stoker’s original and classic Dracula, to odd New Zealand vampire communities all the way to modern occultism. All stories have at least one core element that remain unchanged: vampires drink blood. How could a vampire, a creature essentially human in form and apparent physiology, survive off human blood alone?
While we know about creatures that engage in haematophagy (blood eating), such as the vampire bat, mosquitoes, and even some birds, these animals are small and can survive off the proteins and fats found in blood. However, the first, and most obvious point to be made is that vampires are supposed to be dead. Dead equals no metabolism. No need for nutrition - at least not for your native cells, as in the ones that are you. Since you are more bacteria than human cells, the bacteria will stay alive for a good long time after you die. In fact, they thrive because they have an all-you-can-eat buffet going on in your body. However, we know from vampire lore, that vampires don’t decay; they live for hundreds of years and are apparently also extremely sexy.
Staphylococcus aureus is a vampiric bacteria species, that specifically steals iron human blood. Bacteria use iron to multiply, and since mammalian blood is composed of water, proteins and iron, this is a pretty good source of the latter. In blood, four iron molecules are found on each haemoglobin, the protein that transports oxygen throughout our bodies to keep us alive. The chemical reaction between the iron molecules and the oxygen is what make our blood red. Staphylococcus aureus secrete toxins that cause red blood cells to burst, and then harvest the haemoglobin. It can do this in all mammalian blood, but it prefers human haemoglobin.
The vampires’ teeth are another important physiological adaptation. Most animals that use toxins to sedate their prey, have fangs, like snakes. We can assume, that just like these animals, vampire fangs will secrete substances like anticoagulants (blood thinners), and maybe even a local anaesthetic. Anticoagulants are actually erroneously named blood thinners – they do not affect the viscosity of your blood. Instead, they intercept normal cellular pathways that will plug a wound to stop you from bleeding. This way, vampires will be able to extract more blood from their prey.
If we assume that vampires are basically a walking bacteria house, this not only explains why vampires needto eat blood (the staphylococcus), and why they are particularly partial to human blood, but also why they have fangs. While all this may account for their peculiar eating habits, I still cannot account for tales of vampires becoming unreasonably attractive.