The SARS outbreak in 2002 caused unprecedented devastation and exhibited unusually high rates of morbidity and mortality. For the SARS coronavirus, the transmission vehicle was man. What added to the spread was mans convenient, rapid and affordable ways of travel that caused the virus to spread globally in a short span of time. The SARS coronavirus jumped the species barrier, similar to many recent zoonotic diseases avian flu, swine flu, Nipah and encephalitis virus. It is extremely important for us to understand the enemy before it strikes again in order to be able to combat such outbreaks as and when they occur. Molecular mechanisms that the virus exploits within its host and that result in infection and pathogenesis are the key to being able to block its transmission cycle. In this book, leading scientists from around the globe have discussed molecular aspects of viral entry, replication, viral gene expression, pathogenesis and host immune response against the SARS coronavirus.