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.

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.

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.

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.

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.

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.