Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Viral diseases are very common infections caused by viruses, that spread from person to person when the virus enters the body and begins to multiply. Viral diseases result in a wide variety of symptoms that vary in character and severity depending on the type of infection and other factors, including the person s age and overall health. Learn more about viruses, their types, effects on human health and the most dangerous diseases caused by viruses out there, in the following pages.