Virus refers to a submicroscopic infectious agent that replicates solely within the living cells of an organism. Viruses infect all kinds of life, including animals and plants as well as microorganisms, such as bacteria and archaea. They are found in all the ecosystems that exist on Earth. Virology is a subspecialty of microbiology that studies viruses. A biological preparation that induces active acquired immunity against a specific infectious disease is defined as vaccine. It usually comprises an agent that resembles a disease-causing microorganism. It is commonly manufactured from weakened or deceased forms of the microbe, its toxins, or one of its surface proteins. Antivirals, or antiviral drugs, refer to a class of medications that are used to treat viral infections. These drugs do not destruct their target pathogen but prevent it from growing. Vaccines and antivirals play a significant role in controlling pandemics and eradicating viral diseases. This book is compiled in such a manner, that it will provide in-depth knowledge about viruses. Also included herein is a detailed explanation of the various concepts and applications of antivirals and vaccines. This book is appropriate for students seeking detailed information in this area of study as well as for experts.