key: cord-0036437-zpek8i5e authors: Andersen, Bjørg Marit title: Microbes, Transmission Routes and Survival Outside the Body date: 2018-09-25 journal: Prevention and Control of Infections in Hospitals DOI: 10.1007/978-3-319-99921-0_3 sha: fa0eb946fa56ceb044e441cc774aac335779351a doc_id: 36437 cord_uid: zpek8i5e Microbes like bacteria, virus, parasites and fungi may naturally colonize skin and mucous membranes without any sign of illness, for a longer or shorter period, in all humans, animals, fish, parasites, plants and all other living beings. Some types may be more invasive in human tissue than others. Many microbes are free-living in the environment—in water, soil and air and on equipment—as a part of the normal microbial flora on the Earth. Most of them are not dangerous and live in peaceful symbiosis with other living beings and may also be transferred between living species, from man to animal or man to plants and environment—and vice versa. New and old human pathogenic microbes are increasing all over the world. Some agents, like drug-resistant bacteria and highly pathogenic viruses, are more dangerous than others, and some microbes may cause chronic devastating diseases. Transmission routes depend on the robustness of the microbe in the environment, virulence, infectious dose, anatomical site in the body, etc. Pathogenic microbes are spread by contact, air, water, food, beverages, contaminated equipment and environment and are more seldom vector-borne, by insects or animals. The following chapter is focused on the most frequent pathogenic microbes, their preselected localization in the body, transmission routes and survival in the environment. be more invasive in human tissue than others, partly via mucous membranes and lesions in the skin. Many microbes are free-living in the environment-in water, soil and air and on equipment-as a part of the normal microbial flora on the Earth. Most of them are not dangerous and live in peaceful symbiosis with other living beings and may also be transferred between living species, from man to animal or man to plants and environment-and vice versa [1] [2] [3] [4] [5] [6] . Microbes, like bacteria, may outnumber us by a factor of 10 22 , are heavier than us by a factor of 10 8 , have existed on Earth more than 1000 times longer than us and may undergo 500,000 generations in just one of our generations [7] . The existence of humans and animals is dependent on a rich and active bacterial flora in the gut, participating in the decomposition of food substances to energy and growth. Large amounts of bacteria in the gut (1-2 kg) and on the skin is a normal condition. Humans are releasing microbes into the environment and air wherever they move and-at the same time-are picking up new microbes from the environment. Pathogenic microbes cause illness in most humans if introduced into sterile tissue. "Opportunistic" microbes may cause problems in people with reduced immune defence and/or if large amounts are introduced in sterile tissue. Nonpathogenic microbes nearly never cause illness in humans [1] [2] [3] . Human pathogenic microbes often survive for a long time outside the body-in the environment [1] [2] [3] [4] [5] [6] . Therefore, they are special threats to patients, personnel and visitors in healthcare institutions where there often is an accumulation of infectious diseases. New and old human pathogenic microbes are increasing all over the world. Some agents, like drug-resistant bacteria (methicillin-resistant Staphylococcus aureus, MRSA, multidrug-resistant tubercle bacilli and others) and highly pathogenic viruses (Ebola, SARS and others), are more dangerous than others. Other viral agents may cause chronic devastating diseases like HIV and hepatitis B and C. Microbes may have preselected locations and tissues in the host, like influenza virus, pneumococci and tuberculosis mostly in lungs, hepatitis viruses in the liver and blood, Clostridium difficile and norovirus in the gut, coagulase negative staphylococci on the skin, etc. The transmission routes of microbes are many and different and depend on the robustness of the microbe in the environment, climate and temperature, virulence, infectious dose, etc. Pathogenic microbes are spread by contact, air, water, food, beverages, contaminated equipment and environment and are more seldom vectorborne, by insects or animals. Drug-resistant microbes and/or resistance genes are common on the global food market for humans, animals and fish [8] [9] [10] [11] . In addition, increased mobility, climatic changes, overcrowding, war and disasters, poor hygiene and poor infection control are increasing the transmission rate. 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