This is a very nice sentence structure - try and use it in your own writing. There are quite serious health consequences of a sedentary life in front of a computer - the most obvious being eye strain. Sitting at a computer screen for long periods of time is generally unhealthy, as the individual does not get any exercise. However, it is the social consequences that are the most alarming. The computer and the internet have made direct human contact less and less necessary.
People have the illusion of community through the web, when in fact there is no such community.
Nothing can replace real contact with real people in real streets. We understand others only through what they write in e-mails, which is not the same as reading the subtle changes in emotion on a person's face. We may find in the future that people find human relationships too inconvenient and confusing, and prefer to stay with virtual friends rather than real ones. The question asks only about the dangers, so this student has answered the question well. Be careful that you don't begin writing about the wrong topic.