Viral diseases are caused by viruses, which are small infectious agents that replicate inside living cells of other organisms. These diseases are often highly contagious and can cause a wide range of symptoms, ranging from mild to severe. Some common viral diseases include the flu, common cold, HIV/AIDS, measles, hepatitis, and COVID-19. Research in this area focuses on understanding the mechanisms of viral infection, transmission, and replication, as well as developing vaccines and treatments to prevent and control these diseases. Additionally, researchers study the epidemiology and impact of viral diseases on human populations and work to improve diagnostics and surveillance methods to detect and monitor outbreaks.