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Chapter 21- Infectious Diseases Infectious Diseases- diseases caused by organisms or viruses that enter & multiply within the human body Microorganisms- organisms so small that they can only be seen through a microscope. Pathogens- microorganisms and viruses that cause disease Bacteria-simple, single-celled microorganisms; live in air, soil, food and on the bodies of plants and animals Viruses- the smallest of all pathogens (they are about 100 times smaller than most bacteria Fungi- organisms such as yeasts, molds and mushrooms Protozoans- single-celled organisms that are much larger and more complex than bacteria How Pathogens Are Spread *Infected people *Infected animals *Contaminated objects *Contaminated food, soil or water Defenses Against Disease Physical & Chemical Defenses: your body’s first line of defense against infectious disease Skin- serves as a physical & chemical barrier; surface cells are hard & have no gaps between them; sweat acts as a chemical barrier because it contains acids that kill many bacteria Mucus membranes- secrete a liquid called mucus; mucus traps many pathogens & washes them away Cilia- trap and remove pathogens Saliva/tears- trap pathogens & washes them away Digestive system- chemicals in your digestive system kill many pathogens; the normal motion of the digestive system also moves pathogens out Inflammation: second line of defense; inflammation is your body’s general response to all kinds of injury, from cuts & scrapes to internal damage Phagocytes- blood, other fluids and white blood cells that leak out of enlarged blood vessels to engulf and destroy pathogens Immune System: 3rd and most sophisticated line of defense against pathogens Lymphocytes- white blood cells that carry out most of the immune system functions Immunity- your body’s ability to destroy pathogens that it has previously encountered before the pathogens are able to cause disease T Cells- a type of lymphocytes that perform several functions Killer T cells- destroy any body cell that has been infected by a pathogen Helper T cells-produce chemicals that stimulate other T cells & B cells to fight off infection Suppressor T cells- produce chemicals that “turn off” other immune cell system cells when an infection has been brought under control B cells- lymphocytes that produce antibodies Antibodies- proteins that attach to the surface of pathogens or to the toxins produced by pathogens Lymphatic system-network of vessels that collects fluid from your tissues and returns it to your bloodstream Passive Immunity- immunity acquired by receiving antibodies from a source other than one’s own immune system Active Immunity- immunity that results from either having a disease or from receiving a vaccine Immunizations- injections that cause you to become immune to a disease