How the Flu Works

by Stephanie Watson

Please copy/paste the following text to properly cite this How Stuff Works article:

Watson, Stephanie. "How the Flu Works." 03 November 2004. HowStuffWorks.com. <http://health.howstuffworks.com/flu.htm> 19 July 2008.

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Dr. Whyte's Health Tips: Flu Shots

Dr. Whyte's Health Tips: Flu Shots

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Photo courtesy CDC
Influenza virus

Every winter, like clockwork, the flu returns. It infects millions of us -- about 5 to 20 percent of the U.S. population alone, according to the Centers for Disease Control and Prevention (CDC). Health Canada estimates that 10 to 25 percent of Canadians get the flu each year. It leaves us sniffling, sneezing, coughing, achy and generally feeling miserable for anywhere from a few days to a few weeks.

Although most of us think of the flu as a mild annoyance that we have to deal with each winter, it can actually be a very dangerous disease. The CDC estimates that in the United States alone, more than 200,000 people are hospitalized with the flu or with flu-related complications each year, and more than 30,000 people die from it. Around the world, the flu kills close to a half million people every year.

In this article, we'll find out how people get the flu, what the symptoms are and how to protect yourself.

2004 Vaccine Shortage In 2004, the British company Chiron announced that none of its flu vaccine (Fluvirin®), about 46 million doses, would be available for the 2004-05 flu season. Without Chiron's vaccines, the nation's flu vaccine supply was cut in half. By early October 2004, 30 million doses of the 54 million still available flu vaccines had already been given out. Lines for flu shots snaked around the block outside of supermarkets and health-department offices. To conserve the remaining vaccines for those who needed it most, the CDC asked that only people in high-risk groups get vaccinated.

The rest of the world did not report any shortages, primarily because many countries in Asia and Europe produce their own vaccine supplies and because other countries have different policies governing the administration of flu shots.

Read Press Release: Chiron will not supply Fluvirin® influenza virus vaccine for the 2004-2005 influenza season to learn more.

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Symptom Checker:

Abdominal Bloating Abdominal Fullness Prematurely Afte... Abdominal Lumps Abdominal Mass Abdominal Pain Abdominal Rigidity Abdominal Sounds Abdominal Swelling Abnormal Posturing Abnormal Tooth Shape Abnormally Dark Or Light Skin Absent Menstruation Absent Thirst Agitation Ambiguous Genitalia Ankle Pain Appetite Decrease Appetite Increase Apraxia Arachnodactyly Armpit Lump Autistic Behavior Babinski's Reflex Bed Wetting Belching Binge Eating Blackheads Bleeding Gums Bleeding Into The Skin Blindness Blockage Of Leg Arteries Blood In The Semen Blood in Urine Bloody Or Tarry Stools Bone Pain Or Tenderness Bounding Pulse Bowel Incontinence Breast Lump Breast Pain Breath Odor Breath Sounds Breathing - Slowed Or Stopped Breathing Difficulty Breathing Difficulty When Lying Dow... Breathing That Sounds Abnormal Broad Nasal Bridge Bulging Fontanelles Bullae Chemosis Chest Pain Chills Clammy Skin Claw Foot Claw Hand Cloudy Cornea Clubbing Of The Fingers Or Toes Cold Intolerance Coloboma Of The Iris Comedones Confusion Constipation Contracture Deformity Coughing Coughing Up Blood Crying Excessively in Infants Cyst Decerebrate Posture Decorticate Posture Decreased Consciousness Decreased Urine Output Delayed Growth Delayed or Absent Tooth Formation Depression Diarrhea Difficulty with Urination Dizziness Drooling Drowsiness Dry Eyes Dry Hair Dry Skin Ear Discharge Earache Earlobe Creases Elbow Pain Epicanthal Folds Erection Problems Erosion Excessive Or Unwanted Hair In Women Excessive Thirst Excessive Urination Excessive Urination at Night Excessively Large Fontanelles Eye Burning, Itching and Discharge Eye Pain Eye Redness Eye Slant Eyelid Drooping Eyes That Bulge Face Pain Facial Paralysis Facial Swelling Fainting Fatigue Female Sexual Dysfunction Fever Finger Pain Fingers That Change Color Flank Pain Floating Stools Focal Neurological Deficits Foot Pain Foot, Leg, And Ankle Swelling Foul Smelling Stools Frequent Or Urgent Urination Frontal Bossing Gas (Flatulence) Gastrointestinal Bleeding Genital Sores (Female) Genital Sores (Male) Groin Lumps Groin Pain Gynecomastia Hair Loss Hallucinations Hand Or Foot Spasms Hand Tremors Hearing Loss Heart Murmurs And Other Sounds Heart Palpitations Heartburn Heat Intolerance Heavy, Prolonged, or Irregular Mens... Heel Pain Hemolytic Crisis Hepatomegaly Heterochromia Hiccups Hip Pain Hoarseness Or Changing Voice Hump Behind The Shoulders Hyperactivity Hyperelastic Skin Hypermobile Joints Hyperventilation Hypotonia Impaired Smell Increased Head Circumference Indigestion Infantile Reflexes Insect Bites And Stings Intercostal Retractions Intertrigo Irritability Itching Jaundice (Yellow Skin) Joint Pain Joint Swelling Knee Pain Leg Pain Lichenified Limited Range Of Motion Lordosis Low Nasal Bridge Lower Back Pain Macule Malaise Memory Loss Metopic Ridge Microcephaly Micrognathia Morbid Obesity Morning Sickness Moro Reflex Motormental Retardation Mouth Sores Muscle Aches Muscle Atrophy Muscle Cramps Muscle Function Loss Muscle Twitching Nail Abnormalities Nasal Congestion Nasal Discharge Nasal Flaring Natal Teeth Nausea And Vomiting Neck Lumps Neck Pain Night Blindness Nightmares Nikolsky’s Sign Nipple Discharge Nodules Nosebleed Numbness And Tingling Obesity Opisthotonos Pain During Sexual Intercourse Painful Menstrual Periods Painful Urination Pale or Clay-Colored Stools Paleness Papule Patches Patchy Skin Color Pectus Carinatum Pectus Excavatum Penis Pain Photophobia Pilonidal Dimple Pinna Abnormalities And Low-set Ear... Point Tenderness Polydactyly Polyhydramnios Poor Feeding In Infants Preauricular Tag Or Pit Premenstrual Breast Tenderness and ... Prognathism Pupils That Are Different Sizes Purpura Pustules Rapid Shallow Breathing Rash in a Child Under 2 Years Rashes Rib Cage Pain Ridged Sutures Scales Scrotal Swelling Seizures Sensorineural Deafness Separated Sutures Shin Splints Short Philtrum Short Stature Shoulder Pain Simian Crease Skeletal Limb Abnormalities Skin Blushing/flushing Skin Discoloration Skin Lumps Skin Turgor Sleeping Difficulty Sneezing Snoring Sore Throat Spasticity Speech Impairment (adult) Splenomegaly Splinter Hemorrhages Strange Behavior Stress And Anxiety Striae Subcutaneous Emphysema Sunburn Sunken Fontanelles Supernumerary Nipples Swallowing Difficulty Swallowing Pain Or Burning Sweating Sweating Decrease Swelling Swollen Glands Swollen Gums Taste Impairment Telangiectasia Tenesmus Testicle Lump Testicle Pain Tinnitus Tongue Problems Tooth Discoloration Toothaches Ulcers Uncontrollable Eye Movements Uncontrollable Movement Uncontrolled Or Slow Movement Uncoordinated Movement Unpredictable Or Jerky Movement Urinary Incontinence Urine Discoloration Vaginal Bleeding Between Periods Vaginal Bleeding In Pregnancy Vaginal Discharge Vaginal Itching Vesicles Vision Problems Vomiting Blood Walking Abnormalities Watery Eyes Weak or Absent Pulse Weakness Webbing Of The Fingers Or Toes Weight Gain Weight Loss Wheezing White Spots in the Pupil Whitehead Widely Spaced Teeth Wrinkles Wrist Pain Yawning

Diseases and Conditions Glossary:

ABO Incompatibility AIDS ARDS (acute Respiratory Distress Sy... Aarskog Syndrome Aase Syndrome Abdominal Aortic Aneurysm Abortion - Complete Abortion - Elective Or Therapeutic Abortion - Incomplete Abortion - Infected Abortion - Threatened Abscess Acanthosis Nigricans Achalasia Achilles Tendinitis Achondrogenesis Achondroplasia Acidosis Acne Acoustic Neuroma Acoustic Trauma Acquired Platelet Function Defect Acrodermatitis Acrodysostosis Acromegaly Actinic Keratosis Actinomycosis Acute Adrenal Crisis Acute Arterial Occlusion - Kidney Acute Bilateral Obstructive Uropath... Acute Bronchitis Acute Cerebellar Ataxia Acute Cholecystitis (Gallstones) Acute Cytomegalovirus (CMV) Infecti... Acute Gouty Arthritis Acute HIV Infection Acute Interstitial Nephritis Not NS... Acute Kidney Failure Acute Lymphocytic Leukemia (ALL) Acute Lymphocytic Leukemia In-Depth Acute Mountain Sickness Acute Myeloid Leukemia Acute Nephritic Syndrome Acute Pancreatitis Acute Tubular Necrosis Acute Unilateral Obstructive Uropat... Acute Upper Airway Obstruction Addison’s Disease Adenomyosis Adhesion Adjustment Disorder Adolescent Depression Adolescent Pregnancy Adrenocortical Carcinoma Adrenoleukodystrophy Adult Still's Disease Agammaglobulinemia Age-related Hearing Loss Agoraphobia Agranulocytosis Aicardi Syndrome Albinism Alcohol Withdrawal Alcoholic Cardiomyopathy Alcoholic Ketoacidosis Alcoholic Liver Disease Alcoholic Neuropathy Alcoholism Alcoholism In-Depth Alkalosis Alkaptonuria Allergic Conjunctivitis Allergic Rhinitis Allergic Rhinitis In-Depth Allergic Vasculitis Allergies Allergy To Mold - Dander - Dust Alopecia Areata Alpha-1 Antitrypsin Deficiency Alport Syndrome Alström Syndrome Alveolar Abnormalities Alzheimer’s - Resources Alzheimer’s Disease Alzheimer's Disease Epidemic? Alzheimer's Disease In-Depth Amaurosis Fugax Amblyopia Amebiasis Amebic Liver Abscess Amelogenesis Imperfecta Amenorrhea Amniotic Constriction Bands Amphetamine Or Sympathomimetic Into... Amyotrophic Lateral Sclerosis Anal Fissure Analgesic Nephropathy Anaphylaxis Anaplastic Thyroid Cancer Androgen Insensitivity Syndrome Anemia Anemia - B12 Deficiency Anemia In-Depth Anemia Of Chronic Disease Anencephaly Aneurysm Aneurysm In The Brain Angina Angiodysplasia Of The Colon Angioedema Ankylosing Spondylitis Annular Pancreas Anorchia Anorectal Abscess Anorexia Nervosa Anthrax Antisocial Personality Disorder Anxiety Disorders In-Depth Aortic Arch Syndrome Aortic Dissection Aortic Insufficiency Aortic Stenosis Apert Syndrome Apnea Of Prematurity Apoplexy Appendicitis Arrhenoblastoma of the Ovary Arrhythmias Arterial Embolism Arterial Insufficiency Arteriosclerosis Of The Extremities Arteriovenous Malformation - Cerebr... Arthritis Arthritis - Resources Asbestos-related Pleural Effusion Asbestosis Ascariasis Ascites Aseptic Meningitis Aseptic Necrosis Asherman Syndrome Asperger Syndrome Aspergillosis Aspiration Pneumonia Asthma Asthma And Allergy - Resources Asthma In Adults In-Depth Asthma In Children And Adolescents ... Astigmatism Asymptomatic Bacteriuria Asymptomatic HIV Infection Ataxia-telangiectasia Atelectasis Atheroembolic Renal Disease Atherosclerosis Athlete's Foot Atrial Fibrillation/flutter Atrial Myxoma Atrial Septal Defect Atrophic Vaginitis Attention Deficit Hyperactivity Dis... Attention-deficit Hyperactivity Dis... Atypical Mycobacterial Infection Atypical Pneumonia Aural Polyps Autism Autism - Resources Autoerythrocyte Sensitivity Autoimmune Disorders Autoimmune Hepatitis Autonomic Hyperreflexia Autonomic Neuropathy Avian Influenza Avoidant Personality Disorder Axillary Nerve Dysfunction Back Pain And Sciatica In-Depth Back Strain Treatment Backaches And Stress Bacterial Gastroenteritis Baker’s Cyst Balanitis Barbiturate Intoxication And Overdo... Barrett’s Esophagus Bartholin’s Abscess Bartter Syndrome Basal Cell Carcinoma Basal Cell Nevus Syndrome Basal Ganglia Dysfunction Bassen-Kornzweig Syndrome Bathing Trunk Nevus Becker's Muscular Dystrophy Beckwith-Wiedemann Syndrome Bedwetting Bell's Palsy Benign Ear Cyst Or Tumor Benign Positional Vertigo Benign Prostatic Hyperplasia In-Dep... Beriberi Bezoar Bilateral Hydronephrosis Bile Duct Obstruction Biliary Atresia Biliary Stricture Bipolar Disorder Bipolar Disorder In-Depth Birth-acquired Herpes Birthmarks - Pigmented Birthmarks - Red Bladder Cancer Bladder Outlet Obstruction Bladder Stones Blastomycosis Blastomycosis Skin Lesion Bleeding Disorders Bleeding Esophageal Varices Blepharitis Blind Loop Syndrome Blocked Tear Duct Blood Clots Blount's Disease Body Lice Bone Tumors Borderline Personality Disorder Botulism Bowlegs Brachial Palsy In Newborns Brachial Plexopathy Brain Abscess Brain Herniation Brain Tumor - Adults Brain Tumor - Children Brain Tumors - Primary In-Depth Branchial Cleft Cyst Breast Cancer Breast Cancer In-Depth Breast Infection Breast Milk Jaundice Breath Holding Spell Brief Reactive Psychosis Bronchial Adenoma Bronchiectasis Bronchiolitis Bronchitis Bronchopulmonary Dysplasia Brucellosis Bruxism Bulimia Bullous Pemphigoid Bunions Burkitt Lymphoma Bursitis Byssinosis CMV - Gastroenteritis/colitis CMV - Immunocompromised Host CMV - Pneumonia CMV Esophagitis CMV Retinitis CSF Leak Campylobacter Enteritis Canavan Disease Cancer Cancer - Penis Cancer - Renal Pelvis Or Ureter Cancer - Resources Cancer - Throat Or Larynx Cancer - Vulva Candida Esophagitis Canker Sore Treatment Canker Sores Caplan Syndrome Caput Succedaneum Carbunculosis Carcinoid Syndrome Cardiac Amyloidosis Cardiac Tamponade Cardiogenic Shock Cardiomyopathy Carpal Tunnel Syndrome Carpal Tunnel Syndrome In-Depth Cat Scratch Disease Cataract Cataracts In-Depth Catheter-associated UTI Cavernous Sinus Thrombosis Celiac Disease - Sprue Cellulitis Central Pontine Myelinolysis Central Serous Choroidopathy Central Sleep Apnea Cerebral Hypoxia Cerebral Palsy Cervical Cancer Cervical Cancer In-Depth Cervical Dysplasia Cervical Polyps Cervical Spondylosis Cervicitis Chagas Disease Chalazion Chancroid Chapped Hands Chapped Lips Charcot-Marie-Tooth Disease (heredi... Charley Horse Chediak-Higashi Syndrome Chemical Pneumonitis Chemotherapy Cherry Angioma Chickenpox Chiggers Child Abuse - Physical Child Abuse - Sexual Child Neglect And Psychological Abu... Childhood Disintegrative Disorder Chinese Restaurant Syndrome Chlamydia Chlamydia Infections In Women