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Feeling down? You're not alone.
你有感到很失落嗎？不只你這樣。
If depression seems pretty common, it is.
抑郁看起來很普遍，事實上也是如此。
One in every ten adults suffers from depression.
每十個成年人里就有一個會患有抑郁癥。
And that's what has scientists puzzled.
這讓科學家們感到不解。
If depression is a detrimentalcondition, why is it so rampant in the population?
如果抑郁對身體有害，那么為什么人群中抑郁癥患者如此之多呢？
Some scientists think they may have the answer.
一些科學家認為他們已經找到了答案。
Depression may have an evolutionary benefit.
抑郁也許對進化有好處。
It may improve one's chances of fighting off infection.
它能提高人體免疫力。
Scientists have known that depressed people display an immunoactive condition known as inflammation,
一些科學家發現抑郁癥人群體內會產生免疫活性，也就是炎癥，
even when they are not infected.
即便他們未受到感染時也會如此。
For several years they have been finding that genes linked to depression also affect the function of the immune system.
幾年來，科學家們逐漸發現與抑郁有關的基因也會影響免疫系統的功能。
Stress, which can lead to depression, also modifies the immune system.
同樣會導致抑郁的壓力因素也會改變免疫系統。
These findings suggested that depression was somehow linked to the body's ability to fight disease.
從某種程度上講，這些調查結果表明抑郁與人體免疫力有關。
Some scientists now propose that depression might have had an evolutionary advantage in the days before modern medicine and antibiotics.
一些科學家提出早在現代藥物和抗生素出現之前，抑郁就可能能夠給進化帶來好處。
Infection was a major cause of death in early human history.
感染是人類早期歷史中導致死亡的主要原因之一。
Surviving those infections determined which genes were passed to the next generation.
能夠在這些感染中存活就決定了哪種基因能夠傳遞給下一代。
They suspect that depression helped people, especially children, survive infection.
他們推測抑郁可能會幫助人們特別是兒童抵御感染的侵害。
Fatigue leads toinactivity and less social contact which helps keep infection from spreading.
疲勞會讓人不愿意運動，同時也會減少社交，由此便會阻止感染的傳播。
Shunning food helps avoid food sharing which can also spread infection.
對食物沒胃口就會避免與別人分 享食物，這也能夠避免感染傳播。
Sleeplessness, which is associated with both depression and immune system activation,
失眠與抑郁和免疫系統激活也存在聯系，
may have been a mechanism to keep a person alert to fight off predators after injury.
失眠可能會產生一種讓人在受傷后能對感染保持警覺的機制。
If scientists are correct about this link, it could lead to new treatments for depression.
如果科學家們對這些聯系的猜測正確的話，那么它們將促成新的治療抑郁癥的方法產生。
They are interested in drugs that treat conditions where the immune system attacks the person's ownbody, known as auto immune diseases.
他們對用于治療自體免疫癥的藥物很感興趣。
It is possible that medications known to treat auto immune disease may be successful in treating depression also.
那些已知的用于治療自身免疫性疾病的藥物對治療抑郁癥可能同樣有效。
