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The origin of Infections Slide 2 The infectious diseases of man are usually divided into two large groups. Some diseases affect only men, others affect both man and animals, with man most frequently infected from animals. Slide 3 Every infectious disease has not only characteristic clinical manifestations but also its own specific way of invasion into human body. Such disease as dysentery, which is one of the diseases of intestinal infections, is spread through the intestines and stools. Slide 4 The infections of the respiratory tract compose the second subgroup. During coughing or talking the pathogens are discharged from the infected organism with the mucus from the membranes of the respiratory tract into the air in the form drops. The infection is spread as the air, in which there are drops of mucus with the pathogens in it, is breathed in. the diseases of this subgroup are diphtheria, smallpox, etc. Slide 5 The diseases of the third subgroup are spread through the skin and the mucosa in which the pathogens multiply. In some case it is the skin, in others it is the mucous membrane of the eye. Direct contact and various things may be responsible for spreading the infective agent. Slide 6 The diseases of the fourth subgroup are spread by living insects. The pathogens causing these infections circulate in the blood or lymph and are not discharged from the organism. The insects become infected as they ingest the blood of a diseased man and they are infectious after the pathogen has multiplied in their organism. All these diseases, of which encephalitis is an example, are called blood infections. Slide 7 Slide 8