The present book is designed to cater the needs of BSc Microbiology, Biotechnology and Pharmacy. The basic concept of disease, host-pathogen interaction, diagnosis of disease, and chemotherapy and antimicrobials are discussed concisely for the better understanding of the students and form a source material to the teachers. Different diseases caused by the bacteria and viruses are dealt precisely; fulfilling the requirement of the Undergraduate students of Microbiology. The basic concepts of immunology, antigen-antibody interactions, autoimmunity, hypersentivity and immunity disorders are also covered precisely. The subject matter is written in simple language keeping in view of students' standard and very well-illustrated with neat diagrams. A question bank is given at the end of each chapter.