She had coffee instead of blood – and mostly it didn't get in the way. Her body temperature was kinda high. The average person's core temp is somewhere around 37.4 Celsius. She, her veins full of hot coffee, was naturally much hotter. Around 100 C or thereabouts, but usually a little cooler. Especially when she drank cold milk.
She had a special note in her wallet, explaining that she couldn't receive normal blood transfusions. A doctor once told her that blood and coffee mix like oil and water – this had raised more questions than it answered.
Sometimes she felt blessed to have been born into a dimension where vampires didn't exist. They'd probably hunt her constantly, hankering after the coffee in her veins – like americano with a shot of blood syrup, flavoured by its arterial containers.
She had been bitten by mosquitoes a couple of times, though. They didn't survive long. First, buzzing around wildly like tiny insect Aaron Sorkins, hopped up on coke. Then dead.
Then there was that time she'd gone gorge-walking with the girl scouts. Much younger then. Some years ago. Gorge-walking is where you dress in waterproofs and welly boots and wade along a river. It's supposedly a really fun experience, communing with nature. She didn't think so. More like communing with being wet and cold. And communing with leeches.
Back on dry land, at the campsite, she'd peeled seven fat leeches from her legs – each one swollen up with black coffee. She liked to imagine that each one of the leeches had a really good, life-changing, caffeine-fuelled idea right before they died. Probably not, though. Probably just died, like everything.
Leeches sucking coffee from her legs – friends sucking coffee from glasses in some café – the only real difference was the location. The conversation – or lack thereof – was probably the same.
She couldn't donate her blood to the blood bank, obviously. She maybe could have donated coffee-blood to the local Starbucks, but, weirdly, when she went in to suggest it, no-one seemed keen.
The whole coffee-blood thing had definite downsides. For one, she could never sleep more than 90 minutes at a time. This was pretty annoying when it came to sharing a bed with someone.
Useful, though, for working unsociable hours, so that's what she did. A succession of odd jobs in the dead of night, or the light of day, or the weird afternoon quiet. Hotel night manager. Serving food and coffee (not her own!) at the all-night eatery. Trash collector. Parking attendant. Fast food, fast food, fast food. It all bored her to tears, but what else was she going to do? Lie awake at home all night? And besides, she had to eat somehow.
Speaking of food, there was nothing particularly strange about her diet. Certainly not enough to explain the coffee-blood situation. She didn't drink coffee, though; it felt too much like cannibalism.
Her parents had perfectly normal human blood. As such, her genetic aberration was a mystery. She'd checked the family tree at the library, but that was no help. Little to no information. No blood types, and medical records only went back so far.
Her parents were both coffee-drinkers. They refrained from drinking it in front of her, though. But the minute she turned her back... What could they do? People have to stay awake. People have to get through the day. People have to tolerate their jobs, somehow.
It turns out that one-to-two cups of coffee a day help you function in society, while having near 100% caffeine content in your bloodstream just makes you sober and confused. And a little panicky.