Causes. The virus is spread through saliva. One of the ways a person can catch mono is by kissing someone who has it, so mononucleosis is also called the “kissing disease.” It can also be spread when people share eating utensils, toothbrushes or lipstick. In some cases, doctors can’t discover the cause of the disease, but as stated, nearly everyone harbors the virus. It takes about two to six weeks for symptoms to appear once a person has been exposed to the virus.