The ability of the cells to receive, process and transmit signals with its environment as well as with itself is termed as cell signaling. Extracellular signals are the signals which originate from outside the cells. Various physical agents can be responsible for extracellular signals such as voltage, mechanical pressure, light, temperature, etc. The transformation of a signal into a chemical form marks the beginning of signal transduction. This can either directly activate an ion channel or initiate a second messenger system cascade, which conveys the signal through a cell. This book aims to shed light on some of the unexplored aspects of host-pathogen interactions and the recent researches in this field. It provides significant information to help develop a good understanding of this discipline. For all readers who are interested in this field, the case studies included in this book will serve as an excellent guide to develop a comprehensive understanding.