Mono is a severe illness that can cause debilitating symptoms for weeks on end. Understanding the early symptoms can help you seek medical care early, allowing you to manage your symptoms before they become more severe. Visit AFC Urgent Care for same-day illness diagnosis and treatment with no appointment needed. What is Mono? Mono is a contagious viral infection caused by the Epstein-Barr virus. It is primarily spread from person to person through saliva. Because ...