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    How Infection Works, How Pathogens Make Us Sick — The National Academies

    Searching for the facts about infectious disease? The National Academies, advisers to the nation on science, engineering, and medicine, provide objective information about how infection works, major disease threats, global challenges to fighting disease, and prevention and treatment options.

    What You Need To Know About Infectious Disease


    How Infection Works

    How Pathogens Make Us Sick

    Infection with a pathogen does not necessarily lead to disease. Infection occurs when viruses, bacteria, or other microbes enter your body and begin to multiply. Disease occurs when the cells in your body are damaged as a result of infection and signs and symptoms of an illness appear. The incidence of disease among those infected varies greatly depending on the particular pathogen and individual susceptibility.

    Many of the symptoms that make a person suffer during an infection—fever, malaise, headache, rash—result from the activities of the immune system trying to eliminate the infection from the body.

    In response to infection, your immune system springs into action. White blood cells, antibodies, and other mechanisms go to work to rid your body of the foreign invader. Indeed, many of the symptoms that make a person suffer during an infection—fever, malaise, headache, rash—result from the activities of the immune system trying to eliminate the infection from the body.

    Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function. Our bodies often respond with fever (heat inactivates many viruses), with the secretion of a chemical called interferon (which blocks viruses from reproducing), or by marshaling the immune system’s antibodies and other cells to target the invader. Many bacteria make us sick in the same way that viruses do, but they also have other strategies at their disposal. Sometimes bacteria multiply so rapidly they crowd out host tissues and disrupt normal function. Sometimes they kill cells and tissues outright. Sometimes they make toxins that can paralyze, destroy cells’ metabolic machinery, or precipitate a massive immune reaction that is itself toxic.

    Other classes of microbes attack the body in different ways:

    Trichinella spiralis, the parasitic worm (helminth) that causes trichinosis, enters the body encased in cysts residing in undercooked meat. Pepsin and hydrochloric acid in our bodies help free the larvae in the cysts to enter the small intestine, where they molt, mature, and ultimately produce more larvae that pass through the intestinal wall and into the bloodstream. At that point they are free to reach various organs. Those that reach skeletal muscle cells can survive and form new cysts, thus completing their life cycle.

    Histoplasma capsulatum, a fungus that causes histoplasmosis, grows in soil contaminated with bird or bat droppings. Spores of the fungus emerge from disturbed soil and once inhaled into the lungs germinate and transform into budding yeast cells. In its acute phase, the disease causes coughing and flu-like symptoms. Sometimes histoplasmosis affects multiple organ systems and can be fatal unless treated.

    The protozoa that cause malaria, which are members of the genus Plasmodium, have complex life cycles. Sporozoites, the stage of the parasite that infects new hosts, develop in the salivary glands of Anopheles mosquitos. They leave the mosquito during a blood meal from a human, enter the host’s liver, and multiply. Cells infected with sporozoites eventually burst, releasing another cell form, merozoites, into the bloodstream. These cells infect red blood cells and then rapidly reproduce, destroying the red blood cell hosts and releasing many new merozoites to do further damage. Most merozoites continue to reproduce in this way, but some differentiate into sexual forms (gametocytes) that are taken up by the female mosquito, thus completing the protozoan life cycle.

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    Chapter 23 Communicable Diseases Flashcards

    Start studying Chapter 23 Communicable Diseases. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

    Chapter 23 Communicable Diseases

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    communicable disease

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    a disease that is spread from one living organism to another or through the environment

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    a condition that occurs when pathogens in the body multiply and damage body cells

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    Terms in this set (119)

    communicable disease

    a disease that is spread from one living organism to another or through the environment


    a condition that occurs when pathogens in the body multiply and damage body cells


    a piece of genetic material surrounded by a protein coat


    single-celled microorganisms


    substances that kill cells or interfere with their functions


    plantlike organisms that can cause diseases of the lungs, the mucous membranes, and the skin


    single-celled microorganisms that are larger and more complex than bacteria


    malaria is an example of?


    Microorganisms that enter the body through insect bites


    If your body does not fight off the invaders quickly and successfully, you develop an

    contagious/ infectious diseases

    occur when pathogens enter your body.

    the cold and the flu

    Two of the most common communicable diseases


    live almost anywhere on earth


    Can be harmless, helpful, or cause diseases


    Can often be treated with antibiotics

    Transmitted through Direct Contact Indirect Contact

    Airborne Transmission

    How do diseases spread?


    An organism that carries and transmits pathogens to humans or other animals

    ticks, flies, mosquitoes

    examples of vectors malaria

    vector-borne diseases

    Through contaminated objects, vectors, and contaminated food and water

    List three ways that communicable diseases are spread through indirect contact.

    Something that can be spread from person to person or through the environment

    Define the word communicable.

    A virus is a piece of genetic material surrounded by a protein coat, while bacteria are single-celled microorganisms that live almost everywhere on Earth.

    How is a virus different from bacteria?

    puncture wounds, childbirth, and contact with infected animals

    How do pathogens spread in direct contact?

    Contaminated Objects


    Contaminated Food and Water

    How do pathogens spread thru indirect contact?

    Respiratory infections and hepatitis

    What are the most common communicable diseases?

    respiratory tract

    The passageway that makes breathing possible

    respiratory tract

    Connects the outside world to the inside of your body

    respiratory tract

    Includes: nose, throat, & lungs

    avoid close contact with sick people

    wash your hands often

    avoid touching your mouth, eyes, and nose

    eat right and get physical activity to strengthen your immune system

    abstain from smoking

    How to avoid respiratory infections? (6 steps)

    Colds Influenza Pneumonia Step Throat Tuberculosis

    Most common respiratory infections

    mucous membrane

    The lining of various body cavities, including the nose, ears, and mouth


    The common cold is a viral infection that causes

    influenza or the flu

    a viral infection of the respiratory tract.


    symptoms include: high fever, fatigue, headache, muscle aches, and coughing.


    An infection of the lungs in which the air sacs fill with pus and other liquids

    strep throat

    a bacterial infection spread by direct contact with an infected person or through airborne transmission.

    strep throat

    Symptoms include sore throat, fever, and enlarged lymph nodes in the neck.

    strep throat

    An infection in the throat and tonsils caused by group A Streptococcus bacteria (group A strep)


    a bacterial disease that usually attacks the lungs.


    Symptoms include fatigue, coughing, fever, weight loss, and night sweats.


    a viral infection that causes inflammation of the liver.


    A yellowing of the skin and eyes


    Scarring of the liver

    Hepatitis A

    Usually attacks the digestive system through contact with the feces of an infected person.

    Hepatitis A

    Common symptoms include fever, vomiting, fatigue, abdominal pain, and jaundice.

    Hepatitis B

    Can be spread through sexual contact or contact with an infected person's blood.

    Hepatitis B

    can cause liver failure and cirrhosis.

    Hepatitis C

    Most common blood-borne infection in the US

    Hepatitis C

    Is most often spread by direct contact with needles that are contaminated with infected blood.

    Hepatitis C

    Symptoms: jaundice, dark urine, fatigue, abdominal pain, and loss of appetite. It can lead to chronic liver disease, liver cancer, and liver failure.

    The symptoms of the flu, which include high fever and fatigue, are more serious than the symptoms of the common cold.

    Source : quizlet.com

    How Infection Works

    There is a close connection between microbes and humans. Experts believe about half of all human DNA originated from viruses that infected and embedded their nucleic acid in our ancestors’ egg and sperm cells.

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    What You Need to Know About Infectious Disease.

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    IHow Infection Works

    There is a close connection between microbes and humans. Experts believe about half of all human DNA originated from viruses that infected and embedded their nucleic acid in our ancestors’ egg and sperm cells.

    Microbes occupy all of our body surfaces, including the skin, gut, and mucous membranes. In fact, our bodies contain at least 10 times more bacterial cells than human ones, blurring the line between where microbes end and humans begin. Microbes in the human gastrointestinal tract alone comprise at least 10 trillion organisms, representing more than 1,000 species, which are thought to prevent the gut from being colonized by disease-causing organisms. Among their other beneficial roles, microbes synthesize vitamins, break down food into absorbable nutrients, and stimulate our immune systems.

    The vast majority of microbes establish themselves as persistent “colonists,” thriving in complex communities within and on our bodies. In many cases, the microbes derive benefits without harming us; in other cases, both host and microbe benefit.




    And though some microbes make us sick and even kill us, in the long run they have a shared interest in our survival. For these tiny invaders, a dead host is a dead end.

    The success of microorganisms is due to their remarkable adaptability. Through natural selection, organisms that are genetically better suited to their surroundings have more offspring and transmit their desirable traits to future generations. This process operates far more efficiently in the microbial world than in people. Humans produce a new generation every 20 years or so; bacteria do it every 20 to 30 minutes, and viruses even faster. Because they reproduce so quickly, microorganisms can assemble in enormous numbers with great variety in their communities. If their environment suddenly changes, the community’s genetic variations make it more likely that some will survive. This gives microbes a huge advantage over humans when it comes to adapting for survival.

    Go to:

    Types of Microbes

    There are five major categories of infectious agents: Viruses, bacteria, fungi, protozoa, and helminths.


    Viruses are tiny, ranging in size from about 20 to 400 nanometers in diameter (see page 9). Billions can fit on the head of a pin. Some are rod shaped; others are round and 20 sided; and yet others have fanciful forms, with multisided “heads” and cylindrical “tails.”

    Viruses are simply packets of nucleic acid, either DNA or RNA, surrounded by a protein shell and sometimes fatty materials called lipids. Outside a living cell, a virus is a dormant particle, lacking the raw materials for reproduction. Only when it enters a host cell does it go into action, hijacking the cell’s metabolic machinery to produce copies of itself that may burst out of infected cells or simply bud off a cell membrane. This lack of self-sufficiency means that viruses cannot be cultured in artificial media for scientific research or vaccine development; they can be grown only in living cells, fertilized eggs, tissue cultures, or bacteria.


    Viruses are responsible for a wide range of diseases, including the common cold, measles, chicken pox, genital herpes, and influenza. Many of the emerging infectious diseases, such as AIDS and SARS, are caused by viruses.


    Bacteria are 10 to 100 times larger than viruses and are more self-sufficient. These single-celled organisms, generally visible under a low-powered microscope, come in three shapes: spherical (coccus), rodlike (bacillus), and curved (vibrio, spirillum, or spirochete).

    Most bacteria carry a single circular molecule of DNA, which encodes (or programs) the essential genes for reproduction and other cellular functions. Sometimes they carry accessory small rings of DNA, known as plasmids, that encode for specialized functions like antibiotic resistance. Unlike more complex forms of life, bacteria carry only one set of chromosomes instead of two. They reproduce by dividing into two cells, a process called binary fission. Their offspring are identical, essentially clones with the exact same genetic material. When mistakes are made during replication and a mutation occurs, it creates variety within the population that could—under the right circumstances—lead to an enhanced ability to adapt to a changing environment. Bacteria can also acquire new genetic material from other bacteria, viruses, plants, and even yeasts. This ability means they can evolve suddenly and rapidly instead of slowly adapting.


    E. coli

    Bacteria are ancient organisms. Evidence for them exists in the fossil record from more than 3 billion years ago. They have evolved many different behaviors over a wide range of habitats, learning to adhere to cells, make paralyzing poisons and other toxins, evade or suppress our bodies’ defenses, and resist drugs and the immune system’s antibodies. Bacterial infections are associated with diseases such as strep throat, tuberculosis, staph skin infections, and urinary tract and bloodstream infections.

    Source : www.ncbi.nlm.nih.gov

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