Many illnesses are caused by living things that are too small to see without a microscope. Organisms that cause disease are called pathogens. Diseases caused by pathogens are called infectious diseases. An infectious disease is a disease that is caused by the presence of a living thing within the body. When you have an infectious disease, pathogens have gotten inside your body and caused harm.
In the 1860s, a French scientist named Louis Pasteur showed that microorganisms cause certain kinds of diseases. He also showed that killing the microorganisms could prevent the spread of those diseases. Later, other scientists demonstrated that each infectious disease is caused by a speciﬁc kind of pathogen. For example, one kind of pathogen causes pneumonia, and another kind causes chicken pox.
The four major groups of human pathogens are bacteria, viruses, fungi, and protists. Bacteria are one-celled microorganisms. Some bacterial pathogens damage body cells directly. Strep throat, for example, is caused by a bacterium that invades cells in your throat. Other bacterial pathogens produce a poison, or toxin, that damages cells. The bacteria that cause tetanus produce a toxin that damages the nervous system.
Viruses are tiny particles, much smaller than bacteria. Viruses cannot reproduce unless they are inside living cells. When viruses infect cells, cells are damaged or destroyed in the process. The damaged cells release new viruses to infect other cells.
Fungi and protists may also be pathogens. Athlete's foot is caused by a fungus. The tropical disease malaria is caused by a protist.
Pathogens need food and a place to live and reproduce. Sometimes a human body is a suitable place for a pathogen to meet its needs. Pathogens can spread through contact with either an infected person; soil, food, or water; a contaminated object; or an infected animal.
Pathogens often pass from one person to another through direct physical contact, such as shaking hands. People also spread diseases through indirect contact such as a cough or sneeze. Some pathogens, such as those causing botulism, live in the soil. Other pathogens can contaminate water and food. People can pick up pathogens from objects that have become contaminated. You could become infected by eating food, drinking water, or using silverware contaminated with pathogens. Infected animals can pass pathogens to people by biting them.
The body has three lines of defense against pathogens. In the ﬁrst line of defense, the surfaces of the skin, breathing passages, mouth, and stomach function as barriers to pathogens. These barriers trap and kill most pathogens with which you come into contact.
Skin forms a physical and chemical barrier against pathogens. Mucus and cilia in your breathing passages trap and remove most pathogens. A sneeze or cough can also remove pathogens. Most pathogens that you swallow are destroyed by chemicals in your saliva or by stomach acid.
Pathogens that do get into your body can trigger the inﬂammatory response, the body's second line of defense. In the inﬂammatory response, ﬂuid and white blood cells leak from blood vessels into nearby tissues.
The white blood cells then ﬁght the pathogens. The white blood cells involved in the inﬂammatory response are called phagocytes. A phagocyte engulfs and destroys pathogens by breaking them down. During the inﬂammatory response, the affected area becomes red, swollen, and warm. The inﬂammatory response may also cause a fever. The immune response is the body's third line of defense. The cells of the immune system can distinguish between different kinds of pathogens. The immune system cells react to each kind of pathogen with a defense targeted speciﬁcally at that pathogen. White blood cells that target speciﬁc pathogens are called lymphocytes.
There are two major kinds of lymphocytes-T cells and B cells. A major function of T cells is to identify pathogens by recognizing their antigens. Antigens are molecules that the immune system recognizes as either part of your body or as coming from outside your body.
B cells produce chemicals called antibodies. Each kind of B cell produces an antibody that can bind to only one kind of antigen. When antibodies bind to the antigens on a pathogen, they mark the pathogen for destruction.
Acquired immune deﬁciency syndrome, or AIDS, is a disease caused by a virus that attacks the immune system. The human immunodeﬁciency virus, or HIV, is the only kind of virus known to attack the human immune system directly and destroy T cells. This causes the body to lose its ability to ﬁght disease. HIV can spread from one person to another only if body ﬂuids from an infected person come in contact with those of an uninfected person.
Immunity is the body's ability to destroy pathogens before they can cause disease. The two basic types of immunity are active and passive.
If you've been sick with the chicken pox, you have active immunity to the chicken pox virus. In active immunity, your body has produced the antibodies that ﬁght the pathogens. A person acquires active immunity when his or her own immune system produces antibodies in response to the presence of a pathogen.
Active immunity is produced when the T cells and B cells of a person's immune system help destroy the disease-causing pathogens. After the person recovers, some of the T cells and B cells keep the "memory” of the pathogen's antigen. If that pathogen enters the body again, the memory cells recognize the pathogen's antigen. The memory cells start the immune response so quickly that the person usually doesn't get sick. Active immunity often lasts for many years, and sometimes it lasts for life.
A second way to gain active immunity is by being vaccinated against a disease. Vaccination, or immunization, is the process by which harmless antigens are deliberately introduced into a person's body to produce active immunity. The substance used in a vaccination is called a vaccine. A vaccine usually consists of pathogens that have been weakened or killed.
If you do come down with a disease that is caused by a bacteria, you may be given an antibiotic. An antibiotic is a chemical that kills or slows the growth of bacteria without harming body cells. There are no medications that cure viral illnesses. However, there are some over-the-counter medications that may help you feel more comfortable while you get better.
When people are infected by some pathogens, the people are sometimes given injections that contain antibodies to the pathogen's antigens. This type of protection is called passive immunity. Passive immunity results when antibodies are given to a person-the person's immune system does not make them. A person acquires passive immunity when the antibodies that ﬁght the pathogen come from a source other than the person's body. Passive immunity usually lasts no more than a few months.
A baby acquires passive immunity to some diseases before birth. Antibodies from the mother's body pass into the baby's body
Noninfectious diseases are diseases that are not caused by pathogens in the body. Cardiovascular disease, allergies, diabetes, and cancer are examples of noninfectious diseases.
An allergy is a disorder in which the immune system is overly sensitive to a foreign substance-something not normally found in the body. An allergy develops in response to various foreign substances that set off a series of reactions in the body. An allergen is any substance that causes an allergy. People may be allergic to pollen, dust, molds, some foods, or even some medicines.
Allergens may get into your body when you inhale, eat, or touch them. When lymphocytes encounter an allergen, they produce antibodies that signal body cells to release histamine. Histamine is a chemical that is responsible for the symptoms of an allergy, such as sneezing and watery eyes. Antihistamine drugs interfere with the action of histamine.
Some allergic reactions can create a condition called asthma. Asthma is a disorder in which the respiratory passages narrow signiﬁcantly, causing the person to wheeze and become short of breath.
Your pancreas produces insulin, a chemical that enables body cells to take in glucose from the blood and use it for energy. If you have diabetes, your pancreas fails to produce enough insulin, or the body's cells fail to properly use insulin. As a result, a person with diabetes has high levels of glucose in the blood and may even excrete glucose in urine. The person's body cells, however, do not have enough glucose. People with diabetes may lose weight, feel weak, and always be hungry.
In Type I diabetes, the pancreas produces little or no insulin. People with Type I diabetes must get insulin injections. In Type II diabetes, the pancreas doesn't make enough insulin or body cells do not respond normally to insulin. People with Type II diabetes may be able to control the symptoms of diabetes through proper diet, weight control, and exercise.
Cancer is a disease in which cells multiply uncontrollably, over and over, destroying healthy tissue in the process. As cancerous cells divide over and over, they often form abnormal tissue masses called tumors. A carcinogen is a substance or factor in the environment that can cause cancer.
Surgery, drugs, and radiation are all used to treat cancer. People can reduce their risk of cancer by avoiding carcinogens. They can increase their chance of surviving cancer by having regular medical checkups.
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