Infectious Disease
A-Level Biology Topic 10 8:28 English narration · English + 中文 subtitles burned in
Chapters
Transcript
The animal that kills the most people every year is not the shark or the snake.
每年杀死最多人的动物不是鲨鱼，也不是蛇，而是蚊子。
It is the mosquito. And the strange thing is that the mosquito itself does almost nothing to you. What it carries does.
奇怪的是，蚊子本身几乎不伤害你， 伤害你的是它携带的东西。
A single-celled parasite rides inside the insect, and every time the mosquito feeds, the parasite moves between people without ever crossing the open air.
一种单细胞寄生虫寄居在这只昆虫体内， 每当蚊子吸血，寄生虫就在人与人之间移动，全程不必穿过空气。
In this lesson we look at four diseases, at how we fight them, and at why our best weapon against bacteria is slowly wearing out.
这节课我们看四种疾病，看我们怎样对付它们， 以及为什么我们对付细菌最好的武器正在慢慢失效。
An infectious disease is caused by an organism called a pathogen.
传染病是由一种叫病原体的生物引起的。
A pathogen lives in or on a host and causes it harm.
病原体生活在宿主体内或体表，并对宿主造成伤害。
What makes these diseases different from, say, a broken bone is that they are transmissible. The pathogen can be passed from one person to another.
这类疾病与骨折之类不同的地方在于，它们是可传播的：病原体能从一个人传给另一个人。
For this topic you need three kinds of pathogen.
这个主题你需要掌握三类病原体。
Bacteria, which are cells without a nucleus.
细菌，是没有细胞核的细胞。
Protoctists, which are single-celled organisms with a nucleus.
原生生物，是有细胞核的单细胞生物。
And viruses, which are not cells at all — just genetic material in a coat.
还有病毒，它根本不是细胞——只是包在外壳里的遗传物质。
Four diseases, and for each one the examiner wants the pathogen and how it is transmitted.
四种疾病，每一种考官都要病原体和传播途径。
Cholera is caused by a bacterium, Vibrio cholerae, and it is transmitted in contaminated drinking water or food — contaminated with faeces.
霍乱由一种细菌——霍乱弧菌——引起，通过含有人类排泄物的饮水或食物传播。
Malaria is caused by a protoctist, Plasmodium, and it spreads through the bite of an infected mosquito, which acts as the vector — the carrier.
疟疾由一种原生生物——疟原虫——引起，通过受感染蚊子的叮咬传播，蚊子就是媒介，也就是携带者。
Tuberculosis comes from a bacterium called Mycobacterium, and it travels in tiny airborne droplets from coughs and sneezes, so it spreads fastest where people are crowded together.
结核病来自一种叫分枝杆菌的细菌，随咳嗽和喷嚏产生的微小飞沫传播， 所以在人群拥挤的地方传播最快。
And HIV, the virus that causes AIDS, is a virus.
艾滋病毒是一种病毒。
It spreads through unprotected sex, through infected blood such as shared needles, and from a mother to her baby.
它通过无保护的性行为、受污染的血液（例如共用针头）以及母婴传播。
HIV attacks and destroys certain white blood cells, so over time it weakens the immune system itself — that is what HIV/AIDS does.
艾滋病毒攻击并破坏某些白细胞，所以久而久之它削弱的是免疫系统本身。
Controlling a disease always has three sides.
控制一种疾病总有三个方面。
The biological side is the science of the pathogen itself.
生物学方面是病原体本身的科学。
The social side is people's behaviour and education.
社会方面是人们的行为和教育。
And the economic side is the money, the drugs and the staff a country can actually afford.
经济方面是一个国家实际负担得起的钱、药和人手。
For cholera, the answer is clean water and proper sewage treatment, plus good hygiene and vaccines in some areas.
对霍乱，答案是清洁的水和妥善的污水处理，再加上良好的卫生习惯，某些地区还有疫苗。
For malaria, sleep under nets, drain the still water where mosquitoes breed, spray insecticides and take anti-malarial drugs.
对疟疾，睡在蚊帐里，排掉蚊子繁殖的死水，喷洒杀虫剂，服用抗疟药。
For tuberculosis, find infected people and treat them with a long course of antibiotics, give the vaccine, reduce overcrowding and trace the patient's contacts.
对结核病，找出感染者并用长疗程抗生素治疗，接种疫苗，减少拥挤，追踪病人的接触者。
And for HIV, use condoms and clean needles, test donated blood, and teach people how it spreads.
对艾滋病毒，使用安全套和干净的针头，检测捐献的血液，并教人们它是怎样传播的。
An antibiotic is a drug that kills bacteria, or stops them from growing. Take penicillin.
抗生素是一种杀死细菌或阻止细菌生长的药物。
A bacterium is protected by a strong cell wall, which holds it together against the water constantly pushing in.
就拿青霉素来说。 细菌有一层坚固的细胞壁保护，抵挡不断往里挤的水，把细胞撑在一起。
As the bacterium grows, it has to keep building more of that wall.
细菌长大时，必须不断建造更多的细胞壁。
What penicillin blocks is exactly that building step.
青霉素阻断的正是这个建造步骤。
So the cell gets bigger, but its wall cannot keep up.
于是细胞变大了，壁却跟不上。
Water carries on entering, the weak wall gives way, and the bacterium bursts.
水继续进来，脆弱的壁撑不住，细菌就胀破了。
Notice what makes this safe for you. Your own cells have no cell wall at all, so the drug has nothing to do to them.
注意这为什么对你是安全的：你自己的细胞根本没有细胞壁，所以这药对它们无从下手。
Antibiotics do not work against viruses, and you have to be able to say why.
抗生素对病毒无效，而你必须说得出为什么。
Look at a bacterium.
看看细菌。
It has a cell wall it must keep building.
它有一层必须不断建造的细胞壁。
It has its own ribosomes.
它有自己的核糖体。
It runs its own chemical reactions.
它进行自己的化学反应。
Every one of those is something a drug can attack.
这些每一样都是药物可以攻击的对象。
Now look at a virus.
再看病毒。
It has no cell wall.
它没有细胞壁。
It has no ribosomes of its own.
它没有自己的核糖体。
It has no chemical reactions of its own.
它没有自己的化学反应。
It makes copies of itself inside the host cell, using the host's machinery.
它在宿主细胞内部、借用宿主的机器来复制自己。
So there is simply no bacterial target for the drug to attack.
所以根本没有可供药物攻击的细菌靶点。
When you answer this, name the target that is missing.
回答这道题时，要点明缺少的是哪个靶点。
Writing that viruses are not alive is not the reason, and it earns nothing.
写"病毒没有生命"不是理由，一分也拿不到。
Before we get to the problem, how does a doctor know which antibiotic to give?
在讲问题之前，先说说医生怎么知道该用哪一种抗生素。
There is a simple test you should be able to describe.
有一个你应该会描述的简单实验。
Bacteria from the patient are grown so they cover the whole surface of an agar plate — a lawn, spread on a plate.
取病人身上的细菌，让它们长满整个琼脂平板表面——铺成一层菌苔。
Then small paper discs are laid on top, each one soaked in a different antibiotic.
然后在上面放上小小的纸片，每一片浸了一种不同的抗生素。
Leave the plate to incubate and read it the next day.
把平板放进培养箱，第二天再读结果。
Wherever the drug has diffused out and killed the bacteria, you see a clear ring around the disc with no growth in it.
凡是药物扩散出去、把细菌杀死的地方， 纸片周围就会出现一圈没有细菌生长的透明环。
That ring is what you measure.
要测量的就是这个环。
The bigger the ring, the further the drug worked outwards before it ran out, so the more effective that antibiotic is against this strain.
环越大，说明药物在耗尽之前向外起作用的范围越远， 也就说明这种抗生素对这一菌株越有效。
And notice something important: some discs have no ring at all.
还要注意一件重要的事：有些纸片周围完全没有环。
The lawn has grown right up to the paper, which tells you this strain is already resistant to that drug.
菌苔一直长到纸片边上，这说明这一菌株已经对那种药产生了耐药性。
So one plate can tell a doctor which antibiotic will work, which will work best, and which is already useless — and that last column is the story of the rest of this lesson.
所以一块平板就能告诉医生：哪种抗生素有效、哪种最有效、哪种已经没用了—— 而最后这一栏，正是这节课接下来要讲的故事。
Now the problem.
现在说问题。
Sometimes, purely by chance, a mutation makes one bacterium resistant, which means the antibiotic no longer kills it.
有时候纯粹出于偶然，一个突变让某个细菌产生了耐药性， 也就是抗生素不再能杀死它。
Watch what happens next.
看看接下来会发生什么。
In a population most bacteria are ordinary and a few happen to be resistant.
在一个群体里，大多数细菌是普通的，少数碰巧是耐药的。
Give the antibiotic, and the ordinary ones are killed.
用上抗生素，普通的就被杀死。
That leaves the resistant survivors, with all the food and space to themselves, so they are free to multiply.
剩下耐药的幸存者独占所有食物和空间，于是它们可以自由繁殖。
Before long the whole population is resistant, and the infection becomes very hard, or even impossible, to treat.
不久整个群体都变成耐药的，这种感染就变得非常难治，甚至无法治疗。
Nothing mysterious is happening here.
这里没有什么神秘的事。
This is natural selection, nothing more — the antibiotic is simply the selection pressure, selecting for resistant bacteria.
这就是自然选择，仅此而已——抗生素只是选择压力。
So how do we slow it down?
那我们怎样减缓它呢？
Four things.
四件事。
Only use antibiotics when they are really needed — never for a cold or the flu, because those are viral and the drug cannot touch them anyway.
只在真正需要时才用抗生素—— 绝不要用于感冒或流感，因为那些是病毒性的，药物本来就碰不到它们。
Always finish the whole course, because stopping early leaves the toughest bacteria alive, which is exactly the wrong ones to leave.
一定要吃完整个疗程，因为提前停药会留下最顽强的细菌活着，而那恰恰是最不该留下的。
Use the correct antibiotic for the infection you actually have.
针对你实际得的感染，使用正确的抗生素。
And cut back the heavy use of antibiotics in farming, where the same selection happens on a huge scale.
还要减少农业中大量使用抗生素，那里同样的选择在以巨大的规模发生。
Here is the classic question.
这是经典的题目。
A patient with influenza is prescribed an antibiotic.
一位流感病人被开了抗生素。
Explain why it will not help, and why it is actively harmful.
解释为什么它没有帮助，以及为什么它反而有害。
First, why it cannot help.
首先，为什么它不能帮忙。
Influenza is a virus.
流感是病毒。
It has no cell wall for penicillin to block, no ribosomes of its own, and no metabolism — so the drug has nothing to act on.
它没有细胞壁可供青霉素阻断， 没有自己的核糖体，也没有代谢——所以药物没有可作用的对象。
Second, why it is actively harmful.
其次，为什么它反而有害。
The patient still carries plenty of harmless bacteria, and the drug reaches those.
病人身上仍带着大量无害的细菌，而药物会作用到它们身上。
The susceptible ones die and the resistant ones survive, so the prescription has selected for resistance for no benefit at all.
敏感的死掉，耐药的活下来，于是这张处方毫无益处地筛选出了耐药性。
To get full marks, name the target that is missing.
要拿满分，就要点明缺少的是哪个靶点。
Three things the examiner keeps asking for.
考官反复要的三点。
First, for any named disease, give the pathogen and the route — both of them, every time.
第一，对任何一种指名的疾病，要给出病原体和传播途径——每次都要两样都给。
Cholera, bacterium, water.
霍乱，细菌，水。
Malaria, Plasmodium, mosquito vector.
疟疾，疟原虫，蚊子媒介。
Second, when you explain why an antibiotic fails on a virus, say what is missing: no cell wall, no ribosomes of its own, no metabolism.
第二，解释抗生素为什么对病毒无效时，要说出缺的是什么： 没有细胞壁，没有自己的核糖体，没有代谢。
Third, describe resistance as natural selection. A resistant mutant survives the drug and reproduces, and its offspring inherit the resistance.
第三，把耐药性描述成自然选择：耐药的突变体在药物下存活并繁殖，后代继承了耐药性。
So there it is.
就是这样了。
Four diseases, each with its own pathogen and its own route.
四种疾病，各有自己的病原体和传播途径。
Control that is never only biological.
控制从来不只是生物学的事。
And a drug that works beautifully on bacteria, does nothing to viruses, and is slowly being defeated by the very selection it causes.
还有一种药，对细菌效果极好，对病毒毫无作用， 并且正被它自己造成的选择慢慢击败。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。