Cirrhosis of the liver is a disease. Smoking is not cancer. Drinking is not cirrhosis. Scientists abide by certain rules to identify and treat diseases. If we are to take seriously the assertion that Internet addiction is a treatable disease, it is necessary to inquire as to whether the rules for diagnosis and disease classification are applied and abided by in responsible and meaningful ways.
Diseases are physical.
There are no signs of addiction. They are, first and foremost, referring to a behavior, an activity, that is, something that people do. They log on to the Internet and read words written by other people. They write words to other people. They may engage in this activity at the expense of other activities, such as being with one's wife, being with one's family, going to work, working on the job, sleeping, etc.