Virology is a sub-field of microbiology that deals with the study of viruses, their structure, properties, classification and evolution. It also studies the interactions of viruses with host cells, their modes of infecting hosts for reproduction, the diseases that they cause and their use in therapy and research, among others. Viral classification is an important area of virology. Plant viruses, bacteriophages, fungal viruses and animal viruses are the classifications of viruses based on the host cells that they infect. Humans can only be infected by viruses from other vertebrates. These viruses cause many important infectious diseases such as dengue fever, AIDS, polio, hepatitis, influenza, common cold, rabies, etc. Viruses such as human papillomavirus, and hepatitis B and C viruses contribute to the development of cervical cancer and liver cancer respectively. Therefore, the study of virology is of immense importance in medical science. Our understanding of viruses has undergone rapid development over the past few decades. This book presents the complex subject of human virology in the most comprehensible and easy to understand language. It includes contributions of experts and scientists, which will provide innovative insights into this field.