People who browse the web for medical information often cannot discriminate between scientifically valid information and quackery. Much of the quackery is particularly appealing to readers with no medical background because it is usually written more clearly than scientific papers. Thus, people who rely on the web when attempting to diagnose their medical conditions are likely to do themselves more harm than good.
The assumption required by the argument is that people who attempt to diagnose their medical conditions are likely to do themselves more harm than good unless they rely exclusively on scientifically valid information.