This chapter discusses in details some of the infectious diseases such as mycobacterial diseases, tuberculosis, leprosy, syphilis, bacterial diseases, viral diseases, rickettsial infections, chlamydial infections, parasitic diseases, fungal infections and opportunistic infections. Tuberculosis, also called as Koch’s disease, is a chronic granulomatous disease caused by Mycobacterium tuberculosis. This has been discussed in this chapter in detail ranging from etiology, classification, morphology, clinical features and diagnosis. Bacterial and fungal infections also have been described in detail, on the same pattern for the better understanding of the students. 14 well-labeled figures related to the infectious diseases are also presented in this chapter.