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1、Microbes:Friend or Foe?A Reading AZ Level U Leveled BookWord Count:1,658Visit www.readinga- for thousands of books and materials.www.readinga-Written by Lisa IngIllustrated by Cende HillLEVELED BOOK UUWZMicrobes:Friend or Foe?Level U Leveled Book Learning AZWritten by Lisa IngIllustrated by Cende Hi
2、llAll rights reserved.www.readinga-Photo Credits:Front cover,page 10:Mediscan/Corbis;back cover:C Squared Studios/Photodisc/Getty Images;title page,page 12:Charles Smith/Corbis;page 3:William Radcliffe/Science Faction/Corbis;page 4:Dr.Darlyne A.Murawski/National Geographic Stock;page 5(top left,bott
3、om left):Dr.David Phillips/Visuals Unlimited,Inc.;page 5(top right):Dr.Gopal Murti/Visuals Unlimited,Inc.;page 5(bottom right):Sharon Franklin/Stephen Ausmus/USDA;page 6:ArtToday;pages 9,13(right):Lester V.Bergman/Corbis;page 13(left):Stockbyte/Getty Images;page 14:Tom Le Goff/Digital Vision/Getty I
4、mages;pages 15,20,22(all):Jupiterimages Corporation;page 16(left):Science VU/Visuals Unlimited,Inc.;page 16(right):Dr.F.A.Murphy/Visuals Unlimited,Inc.;page 17:Tatan Syuflana/AP ImagesWritten by Lisa IngIllustrated by Cende Hillwww.readinga-Microbes:Friend or Foe?CorrelationLEVEL UQ4040Fountas&Pinne
5、llReading RecoveryDRA4Microbes:Friend or Foe?Level U 3Table of ContentsIntroduction.4What.Are.Microbes?.5The.Immune.System.7Four.Groups.of.Microbes.9.Bacteria.9.Viruses.11.Fungi.13.Protozoa.13How.Do.Microbes.Spread.Infection?.14Outbreaks.and.Epidemics.15Fighting.Microbes.with.Vaccines.16Antibiotics:
6、.The.Magic.Bullet.19Conclusion.22Glossary.23Index.24Air coming out of the mouth from a sneeze may travel as fast as 120 kph(75 mph).IntroductionAchoo!.You.sneeze.You.cough.Youre.tired.Your.throat.hurts.You.know.from.these.symptoms.that.youre.getting.sickbut.why?.You.might.be.sick.because.of.a.tiny.m
7、icrobea.germ.Long.ago,.no.one.knew.what.made.us.sick.No.one.had.studied.disease.and.health.Many.people.imagined.they.were.cursed,.or.being.punished.for.something.Now,.we.know.that.some.microbes,.or.germs,.can.cause.disease.But.what.are.microbes?.Where.are.they?.Are.all.microbes.bad?.Can.learning.abo
8、ut.microbes.help.us.stay.healthy?Do You Know?George Washington,first president of the United States,caught a throat infection in 1799.His doctors did a common treatment at the time called bloodletting.They took nine pints of blood from his body in one day.The human body holds only 12 pints of blood.
9、Sadly,Washington died from the treatment,not the disease.Doctors used leeches in bloodletting.Microbes:Friend or Foe?Level U 56Louis PasteurModern medicine owes a lot to Louis Pasteur (pass-TUR).He proved the Germ Theory of Disease,which states that contact with harmful microbes is the cause of dise
10、ase.Pasteurs most important discovery was that heating a sealed canister filled with food to a specific temperature for a certain length of time would kill many organisms inside.This heating process is called pasteurization.Today it is used on fresh milk and canned food.Math MinuteTo pasteurize milk
11、,the milk is heated to 145 degrees Fahrenheit(62.8C)for 30 minutes,or to 163 degrees Fahrenheit(72.8C)for 15 seconds.How much quicker in seconds is it to pasteurize milk at the higher temperature?What Are Microbes?Microbes.are.tiny,.living.organisms.We.need.a.strong.microscope.to.see.them.Microbes.l
12、ive.all.around.us.They.are.in.the.air.we.breathe.They.sit.on.everything.we.touch.They.are.on.our.skin.and.shoes.Many.live.inside.our.body.They.are.everywhere!Some.microbes.are.good.for.us.They.help.protect.us.from.disease.Other.microbes.are.harmful.Harmful.microbes.can.make.us.sick.Strep bacteriaFlu
13、 virusYeast fungusOphryoscolex protozoaDifferent kinds of microbesMicrobes:Friend or Foe?Level U 78On.the.outside,.our.skin.is.a.shield.for.our.whole.body.Inside,.our.strong,.white.blood.cells.travel.in.our.blood,.looking.for.harmful.microbes.They.kill.millions.of.harmful.microbes.every.day.Harmful.
14、microbes.can.damage.our.bodys.important.cells.They.spread.an.infection.from.cell.to.cell,.making.us.sick.The Immune SystemOur.immune system.is.like.our.bodyguard.It.protects.us.Its.made.up.of.our.bodys.important.cells,.tissues,.and.glands.The.parts.of.the.immune.system.work.together.as.a.team.The.te
15、ams.job.is.to.protect.us.from.infection.and.disease.Math MinuteSome microbes can double their population every 20 minutes.If you start out with 1 microbe,how many microbes will you have after 4 hours?Hint:Start with 1 microbe at 0 minutes.Our.bodys.cells.join.together.like.building.blocks.to.help.us
16、.grow.We.have.blood.cells,.bone.cells,.stomach.cells,.skin.cells,.and.muscle.cells.To.protect.these.cells,.the.immune.system.tries.to.keep.out.the.troublemaking.microbes.Our.immune.systems.best.weapon.is.its.great.memory.It.remembers.which.microbes.are.good.and.which.are.bad.It.recognizes.and.rememb
17、ers.harmful.microbes.it.has.seen.before.Ie.back.Do You Know?One drop of blood contains between 7,000 and 25,000 white blood cells.These cells attack invading microbes in different ways.Some white blood cells produce antibodies that cancel out the microbes chemicals,while others surround the microbe
18、and destroy it.antibodiesmicrobesMicrobes:Friend or Foe?Level U 910Four Groups of MicrobesBacteriaBacteria.(back-TEER-ee-uh).are.tiny,.simple.organisms.Bacteria.are.everywhere.One.good.type.of.bacteria.lives.in.our.intestines.It.helps.us.digest.the.food.we.eat.Other,.harmful,.bacteria.cause.infectio
19、ns.Strep.throat.and.food.poisoning.are.infections.caused.by.harmful.bacteria.Strep.throat.bacteria.can.cause.a.very.painful,.swollen.throat.and.a.fever.Scientists sometimes grow bacteria on purpose so that they can study them.Here,strep throat bacteria grow in a dish filled with a liquid that provid
20、es food and a safe environment.Salmonella is the bacteria responsible for many cases of food poisoning.The.bacteria.that.cause.food.poisoning.live.on.many.uncooked.foods.Cooking.foods.to.the.right.temperature.kills.bacteria.But.disease-causing.bacteria.also.live.on.kitchen.countertops.and.on.our.han
21、ds.We.should.wash.our.hands.with.soap.before.we.touch.food.Food.poisoning.can.cause.an.upset.stomach,.vomiting,.and.diarrhea.Fun Food FactSeveral types of bacteria help change milk into yogurt and sour cream.These bacteria eat lactose,or milk sugars,and release lactic acid,which curdles the milk and
22、 makes it more solid.The lactic acid gives yogurt and sour cream a tangy,sour flavor.Microbes:Friend or Foe?Level U 1112A.viral.infection.can.make.you.very.sick.Some.viruses.can.cause.death.A.virus.is.able.to.mutate.It.can.change.its.identitylike.putting.on.a.mask.That.can.make.it.harder.for.the.bod
23、y.to.recognize.the.virus.and.fight.it.off.Viruses.cause.diseases.like.the.flu,.chicken.pox,mon.cold.Influenza virus at 295,000 x magnificationVirusesViruses.(VY-russ-es).are.even.smaller.than.bacteria.A.virus.gets.inside.the.cells.of.people.or.other.animals.Then,.it.spreads.an.infection.from.cell.to
24、.cell.SymptomsHeadachesFeverMuscle Aches&TirednessSore ThroatRunny NoseSneezingCoughingChillsCommon ColdNoNone or MildSometimesYesYesYesYesNoFluYesHigh Fever,102104F for daysYesSometimesSometimesSometimesYesYesDo You Have a Common Cold or the Flu?Microbes:Friend or Foe?Level U 1314How Do Microbes Sp
25、read Infection?Infections.can.spread.easily.Many.microbes.that.cause.infections.live.in.body.fluids.These.fluids.include.saliva,.blood,.and.nasal.mucus.Microbes.in.body.fluids.can.be.spread.when.a.person.blows.his.or.her.nose.or.coughs.The.microbes.are.now.on.the.persons.hands.If.we.touch.the.person
26、s.hand,.we.pick.up.the.germs.Coughing.also.puts.germs.into.the.air.We.can.breathe.in.those.germs.Sometimes.we.touch.something.that.the.sick.person.has.already.touched.What.he.or.she.touched.is.contaminated.with.the.germs.on.his.or.her.hands.There.are.also.a.few.diseases.that.can.pass.from.an.infecte
27、d.insect.or.other.animal.to.a.person.Se.from.infected.dogs,.raccoons,.and.skunks.Bacteria.can.be.on.chickens,.birds,.fish,.and.reptiles.Aes.from.sick.birds.and.another.infection.from.tick.bites.Tissues help keep germs from spreading.FungiLike.other.types.of.microbes,.most.types.of.harmful.fungi.are.
28、almost.invisible.Two.infections.caused.by.harmful.fungi.microbes.are.ringworm.and.athletes.foot.Ringworm.causes.a.raised,.itchy,.red.ring.to.appear.on.the.skin.Athletes.foot.attacks.the.bottom.of.feet.and.the.skin.between.the.toes.It.makes.the.skin.itch,.crack,.and.peel.ProtozoaProtozoa.(pro-toe-ZOE
29、-uh).are.also.tiny,.simple.organisms.They.often.live.in.streams,.ponds,.and.dirty.water.Be.careful.about.the.water.you.drink.If.protozoa.get.into.your.body,.they.can.cause.vomiting,.diarrhea,.and.mouth.infections.One.dangerous.protozoa.is.spread.by.the.bite.of.an.infected.mosquito.It.causes.a.seriou
30、s.disease.called.malaria.Malaria.kills.a.million.people.every.year.Mosquitoes can transfer diseases like malaria and West Nile virus from person to person.Athletes foot is caused by a fungal infection of the skin.Microbes:Friend or Foe?Level U 1516Outbreaks and EpidemicsA.disease.can.spread.from.a.f
31、ew.infected.people.in.one.place.to.thousands.of.people.around.the.world.When.a.small.group.of.people.gets.the.same.disease,.its.called.an.outbreak.When.an.outbreak.of.a.disease.spreads.to.a.large.number.of.people,.its.called.an.epidemic.When.an.epidemic.spreads.to.people.all.around.the.world,.its.ca
32、lled.a.pandemic.An.outbreak.of.a.disease.can.spread.within.days.Airplanes,.trains,.and.buses.carry.both.people.and.their.germs.quickly.A.sick.passenger.can.infect.anyone.he.or.she.passes.during.the.trip.When.another.person.gets.infected,.he.or.she.can.carry.the.germs.to.another.place.and.infect.peop
33、le.there.In.the.United.States,.there.is.a.disease-watching.group.called.the.Centers.for.Disease.Control.and.Prevention.(CDC).The.CDC.tracks.infections.around.the.world.Do You Know?When Europeans and Africans carried smallpox microbes to the Americas,none of the Native Americans had ever been exposed
34、 to the disease.They had no immunity to it.As a result,smallpox killed millions.In South and Central America,nine out of ten people in the largest native cities died from smallpox.Fighting Microbes with VaccinesNo.one.likes.to.get.shots,.but.theyre.important.Shots.of.vaccines.are.given.before.you.ge
35、t.sick.They.prevent.a.disease.from.starting.The.job.of.a.vaccine.is.to.help.our.immune.system.learn.to.fight.off.a.harmful.microbe.Most.vaccines.are.made.from.weak.bits.of.a.virus.or.bacteria.The.immune.system.learns.about.that.virus.microbe.from.the.small.amount.in.the.vaccine.It.learns.how.to.figh
36、t.off.the.real.virus.Remember.that.the.immune.system.has.a.great.memory.for.microbes.cowpoxsmallpoxMicrobes:Friend or Foe?Level U 1718Dr.Jenner.wanted.to.find.out.why,.or.how,.this.happened.In.his.test,.he.gave.a.group.of.healthy.people.a.mild.case.of.cowpox.The.cowpox.made.them.feel.slightly.ill.fo
37、r.a.while.But.while.they.were.sick,.their.immune.system.was.learning.about.that.pox.virus.It.built.up.a.defense.against.the.pox.microbe.It.remembered.how.to.fight.off.that.microbe.His.idea.worked.He.used.one.mild.disease.to.fight.off.a.similar,.more.dangerous,.disease.The.people.in.the.cowpox.test.n
38、ever.caught.a.deadly.smallpox.infection.People.could.be.protected.Smallpox.was.destroyed.Dr.Jenners.idea.of.trying.to.prevent.the.spread.of.a.disease.saved.thousands.of.lives.around.the.world.About.seventy-five.years.later,.Louis.Pasteur.discovered.how.to.weaken.microbes.Like.Dr.Jenners.use.of.cowpo
39、x,.his.discovery.led.to.the.use.of.weakened.microbes.to.make.several.types.of.vaccines.These.vaccines.could.fight.specific.diseases.They.could.be.made.in.large.quantities,.so.many.people.could.be.vaccinated.against.a.disease.at.one.time.Almost.all.children.are.given.shots.of.vaccines.before.they.sta
40、rt.school.A.flu.shot.is.a.vaccine.Many.people.get.a.flu.shot.every.year.to.prevent.a.flu.infection.The.idea.of.using.a.virus.to.fight.itself,.or.for.one.disease.to.fight.another.disease,.isnt.new.It.was.first.thought.of.about.200.years.ago,.in.England.Dr.Edward.Jenner.treated.many.cases.of.cowpox,.a
41、.mild.pox.disease.often.caught.by.farm.workers.He.also.treated.many.cases.of.deadly.smallpox.Epidemics.of.smallpox.killed.thousands.of.people.each.year.There.wasnt.a.cure.for.it.Over.time,.he.noticed.that.anyone.who.had.first.caught.cowpox.never.got.the.deadly.smallpox.virus.later.But.why?He.realize
42、d.that.the.two.pox.diseases.must.be.related.to.each.other.Maybe.they.were.like.cousins.in.a.family.But.how.could.he.test.his.idea.that.one.virus.could.be.used.to.stop.another.virus?Vaccines can be given as shots or taken by mouth.Microbes:Friend or Foe?Level U 1920Germs,or harmful microbes,spread ea
43、sily in crowded places.Dont Spread DiseaseWhen youre sick,its important to stay home and rest.This helps your body fight an infection.Staying home also keeps you from infecting other people.Remember that there are simple things we can do to stop germs from spreading.If youre sick,dont share food,cup
44、s,or plates with other people.If you have a cold,carry tissues.When you blow your nose,cover it.When you cough,cover your mouth.Use a tissue,then throw away the tissue in a trash can.Used tissues can infect other people.Remember to cover it and toss it!And remember to wash your hands.Antibiotics:The
45、 Magic BulletScientists.discovered.that.some.helpful.microbes.are.natural.fighters.These.special.microbes.naturally.produce.strong.medicines.called.antibiotics.Anti means.“against.”.These.helpful.microbes.fight.certain.harmful.microbes.and.kill.them.Antibiotics.are.very.particular.and.attack.only.th
46、e.cells.infected.by.harmful.microbes.They.dont.harm.healthy.cells.or.destroy.other,.helpful,.microbes.Your.doctor.might.prescribe.an.antibiotic.to.stop.an.infection.Antibiotics.work.well.But.sometimes,.a.strong.microbe.fights.back.An.antibiotic.may.not.stop.an.infection.for.several.reasons.A.microbe
47、.might.change.itself.just.enough.so.that.the.antibiotic.doesnt.recognize.it.Also,.an.antibiotic.might.fail.because.that.type.has.been.used.too.often.Microbes.may.have.learned.about.it.alreadyand.how.to.resist.it.And.some.antibiotics.have.been.used.in.the.wrong.way.Scientists.keep.trying.to.produce.n
48、ew,.better.antibiotics.to.fight.stronger.microbes.Do You Know?In 1928,Dr.Alexander Fleming discovered that a common bread mold would kill bacteria.This discovery led to the worlds first antibiotic,penicillin,which is still the worlds most widely used antibiotic.Microbes:Friend or Foe?Level U 2122Sta
49、ying HealthyKeep your immune system healthy In order to stay healthy,you must keep your immune system strong.Eat a balanced diet of nutritious foods,including fruits and vegetables,and drink lots of water.Get fresh air,exercise,and enough sleep each night.Preparing food Wash knives and cutting board
50、s with water and soap after cutting raw meat,and never let raw meat touch cooked foods.Do not eat food from dented or swollen cans.Do not eat food that has fallen onto the floor.Keeping clean The most important microbe-fighting action is staying clean with soap and water.Eighty percent of diseases a