and do you have it?

你患有這種病嗎？

Maybe,

也許，

and it's more likely than you'd think.

這病比你想像中的還要厲害。

We're talking about toxoplasmosis,

這就是弓蟲症。

a disease caused by toxoplasma gondii.

這種疾病由弓形蟲引起。

Like all parasites, toxoplasma lives at the expense of its host,

和其它的寄生蟲一樣，弓形蟲吸取宿主的營養，

and needs its host to produce offspring.

生存和繁殖。

To do that, toxo orchestrates a brain manipulation scheme

為此，弓形蟲精心安排了腦部操緃方案。

involving cats,

受到影響的有猫，

their rodent prey,

和他們捕食的囓齒類動物。

and virtually all other birds and mammals,

病毒繼而感染其它鳥類和哺乳類動物，

including humans.

包括人類。

Documented human infections go as far back as ancient Egypt.

最早有文字記載的人類受感染記錄，可追溯到古埃及時代。

We found samples in mummies.

我們在木乃伊身上也發現了弓蟲樣本。

Today, about a third of the world's population is infected,

如今，全球大約有三分之一的人口受到感染。

and most of them never even know it.

大部分人對此並不知情。

In healthy people, symptoms often don't show up at all.

身體健康的人並不會出現受感染症狀。

When they do, they're mild and flu-like.

就算有，也只是類似感冒的輕微症狀。

But those are just the physical symptoms.

但是，那只是身體上的症狀，

Toxoplasma also nestles into our brains

弓形蟲會進駐我們的腦部，

and meddles with our behavior behind the scenes.

在幕後干擾我們的行為。

To understand why, let's take a look at the parasite's life cycle.

不明白？我們來看看寄生蟲的生命週期吧！

While the parasite can multiply in practically any host,

寄生蟲幾乎可以在任何宿主身上裂變繁洐，

it can only reproduce sexually in the intestines of cats.

但弓形蟲只能在猫的腸部進行有性繁殖。

The offspring, called oocysts, are shed in the cat's feces.

它的幼蟲，也叫囊合子，隨猫的糞便排出。

A single cat can shed up to a hundred million oocysts.

一只猫可以排出高達一億的囊合子。

If another animal, like a mouse, accidentally ingests them,

其它動物，像老鼠，不慎吞食了這些糞便，

they'll invade the mouse's tissues and mature to form tissue cysts.

囊合子隨即入侵老鼠的身體組織，成熟後形成胞囊。

If the mouse gets eaten by a cat,

如果猫吃了這隻老鼠，

the tissue cysts become active and release offspring

弓形蟲的胞囊在猫的體內變得活躍，並產下幼蟲。

that mate to form new oocysts,

幼蟲交配後形成新的囊合子。

completing the cycle.

它們的生命就這樣週而復始。

But there's a problem.

問題是：

A mouse's natural desire to avoid a cat makes it tough to close this loop.

老鼠不喜歡接近猫，這是它的天性。

Toxoplasma has a solution for that.

弓蟲症對此有解決辦法。

The parasites invade white blood cells to hitch a ride to the brain

弓形蟲入侵白血細胞，並進入腦部。

where they seem to override the innate fear of predators.

宿主變得無視對天敵的恐懼，

Infected rodents are more reckless and have slower reaction times.

受感染的囓齒類動物動作鲁莽，反應遲緩。

Strangest of all, they're actually attracted to feline urine,

最奇怪的是，它們對猫尿產生好感。

which probably makes them more likely to cross paths with a cat

這讓它們輕易暴露在猫的活動範圍之內，

and help the parasite complete its life cycle.

寄生蟲的生命循環就可以完成了。

How does the parasite pull this off?

它們是怎麼做到的呢？

Although the exact mechanism isn't known, toxo appears to increase dopamine,

精確的機理仍屬未知領域。弓蟲症好像使多巴胺增多，

a brain neurotransmitter that is involved in novelty-seeking behavior.

這是一種使人產生猎奇行為的腦部神經遞質。

Thus, one idea is that toxo tinkers with neurotransmitters,

也許，弓蟲症擾亂腦部神經遞質，

the chemical signals that modulate emotions.

這種化學反應改變了情感。

The result?

結果呢？

Fatal attraction.

這好感可以致命。

But mice aren't the only animals that end up with these parasites,

但是，老鼠並不是惟一因寄生蟲致死的動物。

and that's where humans, and all of toxo's other hosts, come in.

人類，和其它弓形蟲的宿主都是。

We can accidentally ingest oocysts in contaminated water,

誤飲受污染的水，

or unwashed produce,

食用未洗淨的蔬果，

or from playing in sandboxes,

玩堆沙遊戲，

or cleaning out litter boxes.

或清理垃圾時，都會受到感染。

This is behind the common recommendation that pregnant women not change cat litter.

人們常說：孕婦不要清理猫屎箱．這就是原因。

Toxo can cause serious birth defects.

弓蟲症可以引致嬰兒嚴重的先天缺陷。

We can also get toxo from eating undercooked meat

食用未經煮熟的肉類，也有機會感染弓蟲症。

from other animals that picked up some oocysts.

那些動物身上已有弓形蟲幼蟲，

And it turns out that toxo can mess with our brains, too.

當它們長成弓形蟲時，會擾亂我們的腦部運作。

Studies have found connections between toxo and schizophrenia,

研究發現了弓蟲症與精神分裂症之間的相關性。

bipolar disorder,

像躁鬱症，

obsessive compulsive disorder,

強迫性神經官能症，

and aggression.

和攻擊性行為。

It also slows reactions and decreases concentration,

它使人反應遲鈍，專注力下降。

which may be why one study found that people involved in traffic accidents

研究發現，發生交通意外的人，

were almost three times more likely to have toxoplasma.

患有弓蟲症是沒有患病的三倍。

So is toxo manipulating our brains as an evolutionary strategy

弓形蟲逐步適應並控制我們的大腦，

to get predatory cats to eat us?

這會讓捕食成性的猫把我們給吃了嗎？

Or are our brains just similar enough to a rodent's

也許，我們的大腦和囓齒類動物不相伯仲。

that the same neurological tricks that lure them in catch us in the net, too?

同樣的神經官能騙局，誘使他們把我們困在這惡性循環中？

And is toxo the reason so many people love cats and keep them as pets?

是不是因為弓形蟲，才有那麼多人喜歡猫，把牠們當成寶貝?

Well, the jury's still out on that one.

這一問題懸而未決。

Some recent studies even contradict the idea.

一些近期的研究甚至否認這一說法。

Regardless, toxoplasma has definitely benefitted from humans

無論如何，弓形蟲確實從人類中獲益匪淺，

to become one of the world's most successful parasites.

從而成為世上最成功寄生蟲。

It's not just our willingness to let cats on our dining room tables

不只是我們希望猫與我們一同進食，

or in our beds.

或者一同就寢。

Raising livestock and building cities which attract rodents

家畜的飼養和城巿的建設，吸引了囓齒類動物，

has provided billions of new hosts,

為弓形蟲提供了數以萬億計的新宿主。

and you and your cat may be two of them.

你，還有你的猫，就是其中的兩個。