Medical Microbiology is rapidly expanding with tremendous knowledge and exciting discoveries at molecular level. Relevant information of new discoveries has been included in sixth edition of the Textbook of Microbiology. Each chapter has been carefully, updated and revised extensively in relation to classification, taxonomy, pathogenesis, laboratory diagnosis and other aspects of various microorganisms. Beside these, a new chapter on 'Emerging and Reemerging Infections' has been added to update the knowledge. Emerging zoonotic diseases caused by Nipah and Hendra viruses have been described to understand the pathogenesis, laboratory diagnosis and preventive measures of these diseases. A new and innovative chapter 'Essentials of Microbiology at Glance' has been included to revise various aspects of all important microorganisms in no time. Constructive suggestions from students and teachers were also taken into consideration while revising the book. Like the previous editions, I hope the readers will find this edition more informative and up-to-date.