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Biology
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40-1 Infectious Disease
Photo Credit: © Juergen Berger/Max-Plank Institute/Science Photo Library/Photo Researchers, Inc.
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40-1 Infectious Disease
What causes disease?
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Disease
A disease is any change, other than an injury, that disrupts the normal functions of the body.
Disease-causing agents are called pathogens.
Diseases caused by pathogens are called infectious diseases.
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Disease
Some diseases are produced by bacteria, viruses, or fungi.
Others are caused by materials in the environment, such as cigarette smoke.
Still others, such as hemophilia, are inherited.
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6. The Germ Theory of Disease
In the mid-nineteenth century, Louis Pasteur and Robert Koch concluded that infectious diseases were caused by germs.
This idea is now known as the germ theory of disease.
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Koch's Postulates
Koch's Postulates
Robert Koch developed rules still used today to identify microorganisms that cause specific diseases.
These rules are known as Koch's postulates.
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Koch’s Postulates
Koch’s postulates state:
1. The pathogen should always be found in the body of a sick organism and should not be found in a healthy one.
2. The pathogen must be isolated and grown in the laboratory in pure culture.
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Koch’s Postulates
3. When the cultured pathogens are placed in a new host, they should cause the same disease that infected the original host.
4. The injected pathogen should be isolated from the second host. It should be identical to the original pathogen.
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Agents of Disease
Agents of Disease
Pathogens that can cause disease include:
viruses
bacteria
protists
worms
fungi
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Agents of Disease
Viruses
Viruses are tiny particles that invade and replicate within living cells.
Viruses attach to a cell’s surface, insert their genetic material, and take over many of the functions of the host cell.
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Agents of Disease
Viruses can infect nearly every type of organism.
Diseases caused by viruses include the common cold, influenza, and smallpox.
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Agents of Disease
Bacteria
Most bacteria are harmless to humans.
Bacteria that cause disease either:
break down tissues of the organism for food, or
release toxins that harm the body.
Bacterial diseases include streptococcus infections, diphtheria, and botulism.
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Agents of Disease
Protists
Disease-causing protists are transported from person to person by:
mosquitoes (malaria)
insects (African sleeping sickness)
contaminated water supplies (amebic dysentery).
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Agents of Disease
Worms
Flatworms and roundworms cause many human diseases.
Other parasitic worms include Schistosoma, tapeworms and hookworms.
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Agents of Disease
Fungi
Fungi can infect the outer layers of the skin on the feet (athlete’s foot) or the scalp (ringworm).
Other types of fungi infect the mouth, the throat, and even the fingernails and toenails.
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17. How Diseases Are Spread
How are infectious diseases transmitted?
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18. How Diseases Are Spread
Some infectious diseases are spread from one person to another through coughing, sneezing, or physical contact.
Other infectious diseases are spread through contaminated water or food.
Still others are spread by infected animals.
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19. How Diseases Are Spread
Physical Contact
Some infectious diseases can be spread by direct physical contact.
Some dangerous pathogens are spread by sexual contact.
Most diseases spread by indirect contact, such as through the air.
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20. How Diseases Are Spread
Some behaviors can help to control transmission of diseases spread by physical contact.
Cover your mouth when you sneeze or cough.
Wash your hands thoroughly with soap and warm water often.
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21. How Diseases Are Spread
Contaminated Food and Water
Food poisoning is caused by eating food that has pathogens.
Bacteria are always present in uncooked meat. Bacteria grow quickly in warm, partially cooked food, so you should always cook food thoroughly.
Contaminated water also causes disease, especially in areas with poor sanitation and untreated sewage.
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22. How Diseases Are Spread
Infected Animals 
Animals also spread infectious disease.
Animals that carry pathogens from person to person are called vectors.
Malaria, Lyme disease, West Nile virus, and rabies are diseases carried by vectors.
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23. Fighting Infectious Diseases
Antibiotics are compounds that kill bacteria without harming the cells of the human or animal hosts. They work by interfering with cellular processes of microorganisms.
Antibiotics have no effect on viruses.
Antiviral drugs have been developed to fight certain viral diseases.
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24. Fighting Infectious Diseases
Over-the-Counter Drugs
You can buy many medicines without a prescription.
Over-the-counter drugs treat only the symptoms of the disease, not the cause.
The best treatment for most infections includes rest, a well-balanced diet, and plenty of fluids.
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40-1
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40-1
Pasteur and Koch concluded that infectious diseases were caused by
plants.
germs.
animals.
fungi.
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40-1
Ringworm is a skin disease caused by a
roundworm.
flatworm.
bacterium.
fungus.
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40-1
Animals that carry disease from one person to another are called
toxins.
vectors.
pathogens.
parasites.
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40-1
Allan Steere discovered the causative agent of the disease now known as
tuberculosis.
AIDS.
Lyme disease.
African sleeping sickness.
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40-1
Antibiotics kill
bacteria only.
viruses only.
both bacteria and viruses.
both bacteria and protists.
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