Feeling down? You're not alone.
你有感到很失落吗？不只你这样。
If depression seems pretty common, it is.
抑郁看起来很普遍，事实上也是如此。
One in every ten adults suffers from depression.
每十个成年人里就有一个会患有抑郁症。
And that's what has scientists puzzled.
这让科学家们感到不解。
If depression is a detrimentalcondition, why is it so rampant in the population?
如果抑郁对身体有害，那么为什么人群中抑郁症患者如此之多呢？
Some scientists think they may have the answer.
一些科学家认为他们已经找到了答案。
Depression may have an evolutionary benefit.
抑郁也许对进化有好处。
It may improve one's chances of fighting off infection.
它能提高人体免疫力。
Scientists have known that depressed people display an immunoactive condition known as inflammation,
一些科学家发现抑郁症人群体内会产生免疫活性，也就是炎症，
even when they are not infected.
即便他们未受到感染时也会如此。
For several years they have been finding that genes linked to depression also affect the function of the immune system.
几年来，科学家们逐渐发现与抑郁有关的基因也会影响免疫系统的功能。
Stress, which can lead to depression, also modifies the immune system.
同样会导致抑郁的压力因素也会改变免疫系统。
These findings suggested that depression was somehow linked to the body's ability to fight disease.
从某种程度上讲，这些调查结果表明抑郁与人体免疫力有关。
Some scientists now propose that depression might have had an evolutionary advantage in the days before modern medicine and antibiotics.
一些科学家提出早在现代药物和抗生素出现之前，抑郁就可能能够给进化带来好处。
Infection was a major cause of death in early human history.
感染是人类早期历史中导致死亡的主要原因之一。
Surviving those infections determined which genes were passed to the next generation.
能够在这些感染中存活就决定了哪种基因能够传递给下一代。
They suspect that depression helped people, especially children, survive infection.
他们推测抑郁可能会帮助人们特别是儿童抵御感染的侵害。
Fatigue leads toinactivity and less social contact which helps keep infection from spreading.
疲劳会让人不愿意运动，同时也会减少社交，由此便会阻止感染的传播。
Shunning food helps avoid food sharing which can also spread infection.
对食物没胃口就会避免与别人分 享食物，这也能够避免感染传播。
Sleeplessness, which is associated with both depression and immune system activation,
失眠与抑郁和免疫系统激活也存在联系，
may have been a mechanism to keep a person alert to fight off predators after injury.
失眠可能会产生一种让人在受伤后能对感染保持警觉的机制。
If scientists are correct about this link, it could lead to new treatments for depression.
如果科学家们对这些联系的猜测正确的话，那么它们将促成新的治疗抑郁症的方法产生。
They are interested in drugs that treat conditions where the immune system attacks the person's ownbody, known as auto immune diseases.
他们对用于治疗自体免疫症的药物很感兴趣。
It is possible that medications known to treat auto immune disease may be successful in treating depression also.
那些已知的用于治疗自身免疫性疾病的药物对治疗抑郁症可能同样有效。
