Infectious mononucleosis, also known as mono, is an infectious viral disease that most commonly affects young adults. Mono affects the respiratory system, lymphatic tissues and glands in the neck, groin, armpits, bronchial tubes, spleen and liver. The viruses that cause mono are easily passed from person to person by kissing, sharing utensils, sexual contact or through the air (sneezing and coughing). The incubation period is about 10 days in children and 30 to 50 days in adults. Nearly 90 percent of people over age 35 have mono antibodies in their blood, indicating that they had the disease at some point in their lives, even if they did not know they had it.