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Vampire stories have been popping up for centuries,
吸血鬼的故事已流传了几世纪，
and some of them are a lot scarier -- and less sparkly -- than others.
其中一些故事非常恐怖，一些稍显平淡。
The most common stories involve dead people walking the Earth with pale skin, a fear of sunlight, and a thirst for blood
最常见的故事讲的是一些皮肤苍白、害怕阳光且嗜血的活死人
but that myth might actually have some basis in scientific fact. Really.
但是这种谣言可能还是有些科学事实依据的。还真是这样。
Most cultures have some sort of vampire-like story.
多数文化中都有一些和吸血鬼类似的故事。
In Chinese folklore, for example, there are evil spirits that drain your life away,
例如中国民间传说中就有能吸干你生命的恶魔，
and in India, there are legends about demons who used to be people, but weren't cremated properly.
在印度，也有关于恶魔的传说，这些恶魔曾是人类，但死后没有正确的进行火葬。
Mainly, these myths probably came from a deep misunderstanding of how diseases spread.
基本上这些传说可能来自于对于疾病传播的一种深深的误解。
During epidemics like the plague,
在流行病如黑死病期间，
people were understandably terrified and needed an explanation for the apparent randomness of illness.
人们难免会恐惧，他们需要一个说法解释疾病突然爆发原因。
But the vampire myth that's caught on in pop culture has very specific symptoms,
但是流行文化中的吸血鬼传说有着非常明确的症状，
and scientists believe they might be traced back to actual medical conditions.
科学家们认为这有可能与当时的医疗状况有关。
The first possible connection is a group of disorders called porphyria,
第一个可能的联系就是一组疾病——卟啉症，
which are caused by too much of a molecule in your blood called porphyrin.
这种疾病多数是由血液中的一个分子——卟啉引起的。
Your red blood cells mostly contain a massive protein called hemoglobin,
你的红血球多数包含某种大量的蛋白质——血红蛋白，
which moves oxygen and carbon dioxide around your body.
它们能够在你的身体内移动氧气和二氧化碳。
And in the middle of hemoglobin is a ring of carbon, nitrogen, and hydrogen called a porphyrin ring.
在血红蛋白中间是一串碳、氮和氢被称为卟啉环。
Normally, enzymes convert porphyrin into another molecule called a heme group.
一般来讲，酶会将啉环转化为另一种分子即血红素。
But for someone who suffers from porphyria, their body doesn't have enough of one of those enzymes,
但对于那些患有卟啉症的人而言，他们的身体并有足够的酶，
so the porphyrin builds up and causes problems.
因此卟啉不断增强并引起各种问题。
For one, it makes their skin really sensitive to sunlight,
其中一种问题就是它会使患者的皮肤对阳光极为敏感，
and they get painful rashes and blisters… but only when they go outside.
患者会起皮疹和水疱，但只有当他们出门的时候才会这样。
The excess porphyrin also builds up in their teeth and mouth, causing red or brown discoloration.
过量的卟啉也会聚集在他们的牙齿和口腔中，引起红色或棕色的污点。
It can even look a bit like they've been drinking…blood.
可能看起来有点像喝了血一样。
Today, porphyria is treatable, but back in the middle ages, it wasn't very well understood,
如今卟啉症是可以治疗的，但在中世纪这种病还不被理解，
so people could have made up all kinds of stories about it.
所以人们就会编造关于这种病的各种故事。
Most kinds of porphyria are also hereditary conditions,
大多数卟啉症也是遗传病，
so stories could have easily spread about a group of vampires, or one vampire who infected their family.
所以故事就又变成了有一群吸血鬼或是一个吸血鬼传染给了他的家人。
Another likely origin for the myth is a condition called pellagra.
这个传说的另一个可能的起源是糙皮病。
Pellagra is caused by a dietary deficiency of two things -- niacin, also called vitamin B3, and an amino acid called tryptophan.
糙皮病是由缺食性营养不良引起的——一是缺乏烟酸，也称维生素B3，另一个是缺乏氨基酸也称色氨酸。
Normally, your body uses niacin to turn food into energy.
正常情况下，你的身体利用烟酸将食物转化为能量。
And even if you aren't getting enough of it in your diet,
即使在饮食你中没有获取足量的烟酸，
your body will also create niacin from tryptophan, so everything should still run smoothly.
你的身体也会从色氨酸中制造烟酸，这样身体就能够顺利运行。
But when you aren't getting enough of either, things go wrong.
但当你既无法获取烟酸也没有色氨酸时，就要出问题啦。
Around 300 years ago, corn started to replace wheat as the food staple for a lot of Europeans.
约300年前，玉米代替了小麦成为许多欧洲人的主食。
This was great because it grew really well and was cheap, but it also wasn't very nutritious.
这很好因为玉米长得好又便宜，但玉米并不是非常有营养的食物。
In Mexico, where corn had been farmed for centuries before it came to Europe,
在传入欧洲前，墨西哥种植玉米已有几世纪了，
the kernels were treated with a mixture of water and the mineral lime before use.
他们在使用前会用水和矿物酸橙的混合物处理麦粒。
That caused a reaction that released all those healthy vitamins,
这导致了一种反应释放出了所有那些健康的维他命，
but it wasn't a practice Europeans brought home with them.
而欧洲人并没有把这个惯例带入欧洲。
To make things worse, corn also doesn't contain much tryptophan,
更糟糕的是，玉米还不包含足量色氨酸，
so there was nothing for their bodies to convert into niacin.
所以他们的身体中也没有什么物质能够转为烟酸。
No matter how much corn people ate, they couldn't get the vitamins they needed,
不论人们吃了多少玉米，他们都无法获取所需的维他命，
so pellagra started popping up all over the place.
所以糙皮病就开始出现。
The symptoms of pellagra actually look pretty similar to porphyria.
糙皮病的症状事实上和卟啉症非常相似。
Patients are sensitive to sunlight and get terrible rashes if they go outside.
病人们对阳光非常敏感，如果外出便会起严重的皮疹。
Sometimes, the rash gets so bad that the skin starts to degrade over time, looking pale and papery.
有时，皮疹变得非常糟糕，皮肤开始恶化看起来像白纸一样苍白。
Their mouths also turn red, and their tongues start to swell.
他们的嘴巴会变红，他们的舌头开始肿胀。
People are often left with imprints of their teeth in on their tongues,
通常他们的舌头上就剩下牙齿的印记，
which could have started the idea that vampires have large canine teeth.
这也就引发了吸血鬼有大尖牙的想法。
Pellagra can also cause neurons to degenerate in the brain, which can lead to severe mental illness and insomnia.
糙皮病也会引起大脑神经元退化，从而导致严重的精神病和失眠症。
Add in a sunlight allergy, and that might explain why the vampires in stories don't sleep at night.
再加上阳光过敏，这也许可以解释为什么故事中的吸血鬼在夜间不睡觉。
Now, pellagra and porphyria might explain some symptoms,
如今，糙皮病和卟啉症或许也能解释其中一些症状，
but the whole "walking dead" thing might have come from something totally different.
但“行尸走肉”这件事可能源于完全不同的东西。
Some think it comes from a neurological condition called catalepsy.
一些人认为它源于神经系统的疾病，称为强直性昏厥。
Like epilepsy, it affects signaling in the central nervous system.
就像癫痫症，它会影响中枢神经系统的信号。
But instead of the muscle spasms, the person's muscles seize completely, and everything seems to shut down.
但与肌肉痉挛不同，人体的肌肉完全卡住，身体机能似乎完全停工。
They can't move at all, and their heart and respiration rates slow down until it looks like they're actually dead.
他们完全不能动，他的心脏和呼吸速率放缓直到他们像是真的死了一样。
And since doctors back in the day didn't have access to sensitive machines to measure whether someone was alive,
而当时的医生并没有敏感的机器能够检测患者是死是活，
they often thought someone experiencing a cataleptic seizure had died.
他们通常认为患者全身僵硬症后就是死了。
And what did they do with dead people? They buried them.
那他们怎么处理这些死了的人呢？埋起来。
Like you can imagine, if you saw some guy climb out of his coffin,
就像你可以想象，如果你看到某人从棺材里爬起来，
it'd be super hard for him to convince you he was actually alive and not a blood-sucking vampire.
你可能很难相信这个人是活人而不是吸血鬼。
All three of these conditions still exist, but thankfully, they're much more treatable or avoidable now.
所有这三种情况仍然存在，但是幸运的是，如今这些疾病都是可治疗或可避免的。
And at the end of the day, we know they're definitely not anything to be afraid of.
最后，我们知道世上绝对没有什么让人害怕的东西。
Thanks for watching this episode of SciShow, and Happy Halloween!
感谢收看这期的科学秀，万圣节快乐！
If you'd like even more spooky science,
如果你想了解更多令人毛骨悚然的科学知识，
you can watch our episode from last year, where we explain some bloody amazing facts about vampire bats.
你可以看看我们去年的一期节目，关于吸血蝙蝠的惊人真相。
