Part 3

What is a virus? If you've ever had a cold, then you've been infected by a virus. Viruses are the simplest form of life and are much smaller than bacteria. They are responsible for lots of different diseases. Living in bacteria Unlike bacteria, viruses can't make copies of themselves, or reproduce, without help. They have to force the cells of another organism to do the copying for them. A bacteriophage is a type of virus that infects bacteria. The shell of a virus is called a capsid. Instructions for building a copy of the virus are stored in its DNA. These tail fibers help the virus attach to the bacteria cell. Shapes Viruses come in a variety of different shapes and sizes. Here are just a few examples. Adenovirus Adenovirus affects breathing. Its shell is shaped like 20 triangles arranged into a ball. Influenza Influenza, the flu virus, is wrapped in a layer of the infected person's skin. Ebola Ebola is shaped like a curly tube. It is an incredibly deadly virus. Making copies The virus injects its DNA into the bacteria cell. DNA is a really long molecule. It is a bit like an instruction manual because it holds all of the instructions for building copies of the virus. 1 Special molecules inside the bacteria cell blindly follow the instructions held in the virus's DNA. 2 Different parts of the virus are built separately by the bacteria cell. Then they snap together. 3 Once the virus is fully formed, it bursts out, ready to find and infect another cell.

什么是病毒？ 如果你得过感冒，那你就已经被病毒感染过啦。 病毒是最简单的生命形式，比细菌还要小得多。 很多不同疾病都是病毒引发的。 寄生在细菌中 和细菌不同，病毒没办法靠自己复制、繁殖，必须借助外力。 它们得强迫其他生物的细胞帮自己复制新病毒。 噬菌体是一种专门感染细菌的病毒。 病毒的外壳叫做衣壳。 复制新病毒的指令就储存在它的DNA里。 这些尾纤维能帮助病毒附着在细菌细胞上。 形态 病毒有各种各样不同的形状和大小。 这里只是举几个例子。 腺病毒 腺病毒会影响我们的呼吸。 它的外壳是20个三角形拼成的球形。 流感 流感也就是流感病毒，外层包裹着一层来自受感染者的表层物质。 埃博拉 埃博拉病毒是弯弯曲曲的管状。 它是一种致死率极高的病毒。 复制过程 病毒把自己的DNA注入细菌细胞里。 DNA是一种非常长的分子。 它有点像一本说明书，存着复制新病毒的全部指令。 1 细菌细胞里的特殊分子会盲目遵从病毒DNA携带的指令。 2 病毒的不同部件由细菌细胞分别制造出来。 之后再把它们拼接在一起。 3 病毒完全成型后，就会破细胞而出，准备去寻找感染新的细胞了。

Catching a cold The common cold is a viral infection that gives you a runny nose, sore throat, and headache. The average child catches seven colds a year, which is more than any other infection. Passing it on To infect a new person, a cold virus must somehow get into the nose, eyes, or mouth of its next victim. As you'll see below, that's not hard to do! 1 Wiping snot If someone with a cold wipes a runny nose, the cold virus is transferred to their hand. 2 Touching hands Touching someone else's hand will transfer the virus to their hand. 3 Eating food Touching your food transfers the virus from your skin to your snack. 4 Getting sick Once in your mouth, the virus can multiply, infect your cells, and make you sick. 5 Wiping snot With snot on your hands, you will transfer the virus onto anything you touch. 6 The cycle continues The virus can then pass onto its next victim. RHINOVIRUS Colds are caused by more than 200 different viruses. The most common cold virus is the rhinovirus. There is no known cure for the common cold! BREAK THE CYCLE! There are a few simple steps you can take to stop a common cold from spreading. Cover your mouth Sneeze and cough into a tissue or the inside of your elbow, not your hand. Wash your hands Before touching your eyes, nose, or mouth, wash your hands with soap and water. Stay home If you have a cold, stay at home until you're better so you don't pass it on to others. Eat and sleep well Eating healthy foods, such as fruits and vegetables, and getting enough sleep will keep your body in top shape to fight viral infections.

患上感冒 普通感冒是病毒感染，会让你流鼻涕、喉咙痛、头疼。 平均每个孩子一年会得7次感冒，比其他任何感染都多。 病毒传播 感冒病毒要感染新人，得想办法进入下一个受害者的鼻子、眼睛或嘴巴。 你往下看就知道，这一点都不难！ 1 擦鼻涕 如果感冒的人擦了流鼻涕的鼻子，病毒就会沾到他的手上。 2 手部接触 接触别人的手就会把病毒传到对方手上。 3 拿食物 触碰食物时，病毒就会从皮肤传到你的零食上。 4 生病 病毒进入嘴巴后，就会繁殖、感染细胞，让你生病。 5 擦鼻涕 你手上沾了鼻涕，就会把病毒传到任何你碰过的东西上。 6 循环继续 病毒接下来就会传染给下一个受害者了。 鼻病毒 引发感冒的病毒有200多种不同类型。 最常见的感冒病毒就是鼻病毒。 目前普通感冒还没有已知的治愈方法！ 打破传染循环！ 有几个简单的方法可以阻止普通感冒传播。 捂住口鼻 打喷嚏咳嗽对着纸巾或手肘内侧，不要对着手。 勤洗手 碰眼睛、鼻子或嘴巴前，用肥皂和清水把手洗干净。 待在家里 如果你感冒了，就待在家里直到康复，别传染给别人。 健康饮食好好睡觉 多吃水果蔬菜这类健康食物，睡足觉，能让你的身体保持最佳状态对抗病毒感染。

Fighting a virus Your body fights viruses like it fights bad bacteria: with white blood cells. But did you know that your body can learn from an infection? Or that your doctor can give you a head start on fighting the next one? Memory cells If you've had the measles virus before, you won't ever catch it again. This is thanks to special white blood cells called memory cells. They remember what your body needs to do to defeat the virus if it shows up again. 1 When an unfamiliar virus enters your body, your immune system isn't properly prepared. The virus spreads and makes you feel sick. 2 Once the infection is detected by your immune system, white blood cells are sent in to do battle with the virus and learn its weaknesses. 3 Your white blood cells figure out how to defeat the virus. Your immune system is victorious! You start to feel better. 4 Most of the white blood cells die now that their work is done, but some stick around. These are the memory cells. They hold on to a memory of the virus so they know how to defeat it if it comes back in the future. Vaccines Doctors can inject you with a vaccine, which contains a weak version of a virus. Your white blood cells easily defeat the weakened virus, making new memory cells in the process. These cells will protect you if the real virus shows up.

对抗病毒 你的身体对抗病毒和对抗有害细菌的方式一样：靠白细胞。 但你知道吗，你的身体能从感染里积累经验哦？ 医生还能帮你提前做好准备，对抗下一次病毒侵袭哦？ 记忆细胞 如果你之前得过麻疹，就再也不会得第二次了。 这都多亏了叫做记忆细胞的特殊白细胞。 它们记住了击败病毒的方法，病毒再来的时候就知道该怎么做了。 1 陌生病毒刚进入身体时，免疫系统并没有做好准备。 病毒扩散，你就会感觉不舒服生病了。 2 免疫系统发现感染后，就会派出白细胞和病毒战斗，还会摸清它的弱点。 3 白细胞会找出击败病毒的方法。 免疫系统大获全胜！ 你开始慢慢好起来。 4 任务完成后，大部分白细胞会消失，但还有一部分留了下来。 这些就是记忆细胞。 它们记住了病毒的信息，未来病毒再来时，就知道该怎么击败它了。 疫苗 医生会给你注射疫苗，疫苗里含有毒性弱化的病毒。 你的白细胞能轻松打败弱化的病毒，还会在过程中产生新的记忆细胞。 这些细胞会保护你，真正的病毒来袭时就能击退它了。

Deadly tricks Viruses multiply by taking over the cells of an organism. However, to survive for a long time, they must jump from one organism to the next. Some viruses have developed tricks to help them make the leap. Rabies The rabies virus spreads when an infected animal, such as a dog, bites another creature. Amazingly, the infection makes the dog angrier and more likely to bite. This sneaky trick is what helps the virus spread. 1 Initial infection A dog with rabies bites another dog. The virus passes from the saliva of the first dog into the bloodstream of the second. 2 Reaching the brain The virus multiplies and travels through the dog's body to its brain. Once in the brain, the virus causes swelling. 3 Bad behavior Changes to the dog's brain cause its behavior to change. The dog becomes more aggressive and more likely to bite. 4 Dangerous saliva The virus travels down to the mouth, where saliva is made. The saliva the dog produces now contains the rabies virus. The cycle starts again when the dog bites another dog. CAN VIRUSES GET WEAKER? When a virus jumps from an animal to a human, it can cause serious illness. But viruses become less deadly over time. A virus weakens as our immune systems get used to it and become better at attacking it. Zoonotic diseases Though it is rare in humans, rabies is a zoonotic disease. This means it is a disease that can jump from an animal to a human. Zoonotic diseases are among the worst diseases in the world. Here are a few examples. Avian flu Avian flu, or bird flu, is a type of flu that usually spreads between birds. When the virus spreads to a human, it can be deadly. West Nile fever This virus is common in birds, such as crows. It can spread to humans via mosquito bites. Many infected people don't have any symptoms, but for some the virus causes illness. Ebola This life-threatening disease causes a high fever and bleeding in humans. Scientists believe the virus comes from bats. Cat-scratch disease Bacteria can cause zoonotic diseases, too. Cat scratch disease occurs when bacteria break into a human's body by a cat's scratch. This disease causes swelling and achy muscles.

致命诡计 病毒通过侵占生物体的细胞完成繁殖。 不过，想要长期存活，它们必须从一个生物体跳到另一个生物体身上。 有些病毒还进化出了帮自己完成这场跳跃的诡计。 狂犬病 当染病的动物（比如狗狗）咬伤其他生物时，狂犬病毒就会传播开来。 神奇的是，这种感染会让狗狗变得更易怒，也就更容易咬人。 这个狡猾的诡计正好帮助病毒扩散。 1 初始感染 一只患狂犬病的狗咬伤了另一只狗。 病毒从第一只狗的唾液，进入了第二只狗的血液中。 2 抵达大脑 病毒不断繁殖，顺着狗狗的身体一路来到大脑。 病毒进入大脑后，就会引发脑组织肿胀。 3 行为改变 狗狗的大脑被改变后，行为也跟着变了。 狗狗变得更具攻击性，也就更容易咬人。 4 危险唾液 病毒一路下行来到负责制造唾液的口腔。 现在狗狗分泌的唾液里已经含有狂犬病毒了。 当这只狗再咬伤另一只狗，传播循环就重新开始了。 病毒的毒性会变弱吗？ 当病毒从动物跳到人类身上时，会引发严重的疾病。 不过随着时间推移，病毒的毒性会越来越弱。 当我们的免疫系统适应了病毒，变得更擅长对抗它时，病毒的毒性就减弱了。 人兽共患病 虽然人类很少感染狂犬病，但它属于人兽共患病。 意思就是这种疾病可以从动物传染给人类。 人兽共患病是全世界最凶险的疾病之一。 这里举几个例子。 禽流感 禽流感也叫鸟流感，是一种通常在鸟类之间传播的流感。 这种病毒传染给人类后，可能会致命。 西尼罗河热 这种病毒常见于乌鸦这类鸟类。 它可以通过蚊子叮咬传染给人类。 很多感染者不会出现任何症状，但对部分人来说，这种病毒会引发疾病。 埃博拉 这种可致命的疾病会让人类发高烧、体内出血。 科学家认为这种病毒来源于蝙蝠。 猫抓病 细菌也会引发人兽共患病。 当猫咪抓伤人，细菌就会顺着伤口进入人体，引发猫抓病。 这种疾病会导致身体肿胀、肌肉酸痛。

Plant viruses It's not just animals that get sick from viruses. All living things can suffer a virus attack, including plants. Because plants don't move from one place to another, plant viruses need to find other ways to spread from one victim to the next. Sneaky scents The cucumber mosaic virus has a clever way of spreading from plant to plant. It tricks the infected plant into making a powerful odor. The smell attracts plant-eating insects called aphids, who are drawn to the sweet scent every time. 1 What's that smell? An aphid catches the strong scent of a nearby plant. It smells lush and full of delicious nutrients. 2 Flying toward food Aphids swarm to the plant, ready for a yummy meal. When they get there, they take big bites. 3 Gross! The plant is infected with a virus. When the aphids eat it, they also eat some of the virus—and it doesn't taste too good! 4 Spreading the virus Disappointed by the taste, the aphids search for a better meal, taking the virus with them. When they find another plant and take a bite, the virus will spread. TULIP TROUBLE Tulip petals are normally one color, but the tulip breaking virus causes them to grow with white stripes that "break" up the color. It may look cool, but this virus is harmful to the flower. CUCUMBER MOSAIC VIRUS The capsid, or shell, of the cucumber mosaic virus is shaped kind of like a soccer ball. It is made of 12 five-sided pentagons and 20 six-sided hexagons.

植物病毒 不只有动物会被病毒感染生病。 所有生物都可能遭遇病毒攻击，植物也不例外。 因为植物不会移动，所以植物病毒得想其他办法从一个宿主传到另一个宿主。 狡猾的气味 黄瓜花叶病毒有一种聪明的传播方法。 它会诱骗被感染的植物释放出一种浓烈的气味。 这种气味会吸引叫做蚜虫的植食昆虫，蚜虫每次都会被这种甜香吸引过来。 1 那是什么味道？ 一只蚜虫闻到了附近植物飘来的浓烈气味。 闻起来这株植物茂盛又充满美味的养分。 2 飞向食物 蚜虫成群飞向这株植物，准备享用美餐。 它们抵达后，就大口咬了起来。 3 真恶心！ 这株植物已经被病毒感染了。 蚜虫吃叶子的时候，也吃下了不少病毒——而且味道可不太好！ 4 传播病毒 蚜虫对味道很失望，就带着病毒去找更好的食物了。 当它们找到另一株植物咬下一口时，病毒就传播开了。 郁金香的麻烦 郁金香花瓣通常是纯色的，但郁金香碎色病毒会让花瓣长出白色条纹，“打破”原本的纯色。 它看起来可能挺酷，但这种病毒对花朵有害。 黄瓜花叶病毒 黄瓜花叶病毒的衣壳也就是外壳，形状有点像足球。 它由12个五边形和20个六边形组成。

What are fungi? There are lots of different types of fungus, but they all have something in common: They get their nutrients from other organisms, dead and alive, by oozing fluid onto them and then sucking the nutrients back up. Mold A lot of microbes are made of one cell, but mold, a type of fungus, is different. A few cells stick together to form threads that look like branches. They use these threads to feed on nutrients, like those in this piece of fungus-covered bread. Funky fungi You can find different types of fungus, such as molds, yeasts, and mushrooms, growing anywhere it is warm and damp, including on our bodies! Fungal infection Some fungi grow on our bodies and cause diseases like athlete's foot. They thrive in the moist conditions inside sweaty shoes. Dried yeast Yeast is a fungus used as an ingredient in baking. It makes bubbles of carbon dioxide gas that help make air pockets in foods such as bread. Mushrooms Some fungi feed unseen underground until they are ready to reproduce. Then they pop out onto the surface as mushrooms. FEELING CRABBY To be considered a fungus, a microbe's cell walls must be made of a material called chitin. Chitin is the same substance that crab shells and insect exoskeletons are made of.

什么是真菌？ 真菌有许多不同的种类，但它们有一个共同点：它们从其他生物（不论死的还是活的）获取养分，方法是先向对方渗出液体，再把养分吸收回去。 霉菌 很多微生物都只由一个细胞构成，但霉菌作为真菌的一种，却不一样。 数个细胞粘在一起，长成类似树枝的细丝。 它们用这些细丝摄取养分，就像这块长满真菌的面包上的养分一样。 百变真菌 你能在任何温暖潮湿的地方找到不同种类的真菌，比如霉菌、酵母菌和蘑菇，就连我们的身体上也有它们的踪迹！ 真菌感染 有些真菌生长在我们身上，会引发脚气这类疾病。 它们在汗湿鞋子里的潮湿环境中生长得格外旺盛。 干酵母 酵母菌是一种真菌，是烘焙时会用到的原料。 它会产生二氧化碳气泡，在面包这类食物中形成气孔。 蘑菇 有些真菌会在地下悄悄汲取养分，直到准备繁殖。 之后它们就会钻出地面，变成我们看到的蘑菇。 神奇小知识 微生物要被归为真菌，它的细胞壁必须由一种叫作几丁质的物质构成。 几丁质就是构成螃蟹壳和昆虫外骨骼的同一种物质哦。