Viruses--The aliens among us（part-2）

Viruses cause pandemics. They also shape the world

As far as anyone can tell, viruses—often of many different sorts—have adapted to attack every organism that exists. One reason they are powerhouses of evolution is that they oversee a relentless and prodigious slaughter, mutating as they do so. This is particularly clear in the oceans, where a fifth of single-celled plankton are killed by viruses every day. Ecologically, this promotes diversity by scything down abundant species, thus making room for rarer ones. The more common an organism, the more likely it is that a local plague of viruses specialised to attack it will develop, and so keep it in check.

powerhouse n.   /ˈpaʊəhaʊs/  1.a group or an organization that has a lot of power 强大的集团（或组织） 2.a person who is very strong and full of energy 精力充沛的人；身强力壮的人

Relentless  adj. /rɪˈlentləs/  

1.not stopping or getting less strong 不停的；持续强烈的；不减弱的 •her relentless pursuit of perfection 她对完美的不懈追求

2.refusing to give up or be less strict or severe 不放弃的；严格的；苛刻的；无情的 •a relentless enemy 残酷的敌人

prodigious  adj. /prəˈdɪdʒəs/  ( formal ) very large or powerful and causing surprise or admiration 巨大的；伟大的

Slaughter   /ˈslɔːtə(r)/  n. 屠宰；大屠杀；（非正式）彻底击败; v. 屠宰（动物）；（尤指大量）屠杀（人、动物）；（非正式）彻底击败（对手）

Mutate  v. /mjuːˈteɪt/  

1.to develop or make sth develop a new form or structure, because of a genetic change （使）变异，突变 •mutated genes 发生变异的基因

2. to change into a new form 转变；转换 •Rhythm and blues mutated into rock and roll. 节奏布鲁斯演变成为摇滚乐

plankton  /ˈplæŋktən/  浮游生物        scythe /saɪð/ n. 长柄大镰刀；钐刀 vt. 用大镰刀割

This propensity to cause plagues is also a powerful evolutionary stimulus for prey to develop defences, and these defences sometimes have wider consequences. For example, one explanation for why a cell may deliberately destroy itself is if its sacrifice lowers the viral load on closely related cells nearby. That way, its genes, copied in neighbouring cells, are more likely to survive. It so happens that such altruistic suicide is a prerequisite for cells to come together and form complex organisms, such as pea plants, mushrooms and human beings.

Propensity  /prəˈpensəti/  

~ (for sth)~ (for doing sth)~ (to do sth) ( formal ) a tendency to a particular kind of behaviour （行为方面的）倾向；习性 • He showed a propensity for violence. 他表现出暴力倾向。

evolutionary stimulus : 进化刺激

Altruistic /ˈæltruːˌɪstɪk/ 利他的        altruistic suicide 利他性自杀

prerequisite  n. /ˌpriːˈrekwəzɪt/  

~ (for/of/to sth) ( formal ) something that must exist or happen before sth else can happen or be done 先决条件；前提；必备条件

pea plants：豌豆植物

For example, one explanation for why a cell may deliberately destroy itself is if its sacrifice lowers the viral load on closely related cells nearby. That way, its genes, copied in neighbouring cells, are more likely to survive.

这段话的大意是：例如，一种解释细胞为何自毁？细胞的自毁会降低周边细胞的病毒载量。从而提升其被临近细胞复制基因的存活率。

The other reason viruses are engines of evolution is that they are transport mechanisms for genetic information. Some viral genomes end up integrated into the cells of their hosts, where they can be passed down to those organisms’ descendants. Between 8% and 25% of the human genome seems to have such viral origins. But the viruses themselves can in turn be hijacked, and their genes turned to new uses. For example, the ability of mammals to bear live young is a consequence of a viral gene being modified to permit the formation of placentas. And even human brains may owe their development in part to the movement within them of virus-like elements that create genetic differences between neurons within a single organism.

genome n.   /ˈdʒiːnəʊm/  ( biology 生 ) 基因组；染色体组

Descendant  /dɪˈsendənt/  n. 后裔，子孙；（由过去类似物发展来的）派生物；（机器等）后继型产品 adj. 下降的；祖传的

in turn be hijacked：反过来被挟持 

hijack  /ˈhaɪdʒæk/ vt. 抢劫；揩油vi. 拦路抢劫 n. 劫持；威逼；敲诈

placenta  n. /pləˈsentə/ 胎盘     neuron n.  /ˈnjʊərɒn/ 神经元

the ability of mammals to bear live young is a consequence of a viral gene being modified to permit the formation of placentas.大意是哺乳动物能够生育幼崽是病毒基因被宿主修改，从而允许胎盘形成的结果。

原文：经济学人9月刊