Infectious diseases are disorders caused by microorganisms, that impair a person’s health. In many cases, infectious diseases can be spread from person to person, either directly (e.g., via skin contact) or indirectly (e.g., via contaminated food or water) and some are transmitted by insects or other animals.   

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An infectious disease can differ from simple infection, which is the invasion of and replication in the body by any of various microorganisms - including viruses, bacteria, fungi, and parasites—as well as the reaction of tissues to their presence or to the toxins that they produce. When health is not altered, the process is called subclinical infection. Thus, a person may be infected but not have an infectious disease. This principle is illustrated by the use of vaccines for the prevention of infectious diseases.  For example, a virus such as that which causes measles may be attenuated (weakened) and used as an immunizing agent. The immunization is designed to produce a measles infection in the recipient but generally causes no discernible alteration in the state of health. It produces immunity to measles without producing a clinical illness (an infectious disease.) 

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The most important barriers to the invasion of the human host by infectious agents are the skin and mucous membranes (the tissues that line the nose, mouth, and upper respiratory tract). When these tissues have been broken or affected by the earlier disease, invasion by infectious agents may occur.  These infectious agents may produce a local infectious disease, such as boils, or may invade the bloodstream and be carried throughout the body, producing generalized bloodstream infection (septicemia) or localized infection at a distant site, such as meningitis (an infection of the coverings of the brain and spinal cord). Infectious agents swallowed in food and drink can attack the wall of the intestinal tract and cause local or general disease. The conjunctiva, which covers the front of the eye, may be penetrated by viruses that cause a local inflammation of the eye or that pass into the bloodstream and cause severe general disease, such as smallpox. Infectious agents can enter the body through the genital tract, causing acute inflammatory reactions in the genital and pelvic organs or spreading out to attack almost any organ of the body. Even before birth, viruses and other infectious agents can pass through the placenta and attack developing cells, so that an infant may be diseased or deformed at birth. 

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From conception to death, humans are targets for attack by multitudes of other living organisms, all of them competing for a place in the common environment. The air people breathe, the soil they walk on, the waters and vegetation around them, the buildings they inhabit and work in, all can be populated with forms of life that are potentially dangerous. Domestic animals may harbor organisms that are a threat, and wildlife teems with agents of infection that can afflict humans with serious disease. However, the human body is not without defenses against these threats, for it is equipped with a comprehensive immune system that reacts quickly and specifically against disease organisms when they attack. Survival throughout the ages has depended largely on these reactions, which today are supplemented and strengthened by the use of medical drugs.