Publisher Summary This chapter describes the causes and effects of infection. While bacteria are responsible for many diseases of man and animals, it is, however, important to recognize that relatively few bacteria are capable of producing disease; the majority are unable to multiply in the tissues and are readily disposed of by the normal defence mechanism of the body. Bacteria have been grouped into two arbitrary classes -the parasites which grow upon living tissues and the saprophytes which multiply on dead organic matter. The differentiation of these two classes is not absolute, as there is considerable overlapping by members of both groups. Another tentative but convenient classification is the sub-division of bacteria into those able to produce disease-the pathogenic forms, and those without this capacity-the non-pathogenic forms.