Book excerpt: Viral diseases pose a severe threat to public health globally with many pathogenic viruses circulating, and many more might be yet to emerge. Viral infections have become an inevitable event in our daily lives however people may not know much about their causal pathogens. The ongoing pandemic of the coronavirus disease 2019 (COVID-19) raises another alarm that our fight against viral pathogens is far from ending, and our understanding of the complex interactions between the viral invaders and the human host remains limited.