In France, 8% of the population was Muslim as of 2010, but an Ipsos-Mori poll in 2014 found that the average respondent thought the figure was 31%. In Belgium, a 6% Muslim population translated into the perception of a 29% Muslim population.
A possible explanation for this is the fact that the terrorist groups have been so closely affiliated with Islam, even though the vast majority of Muslims have no sympathy for their cause. The combination of natural xenophobia combined with a terrorist fear skews perceptions. George Friedman puts it this way:
When we see pictures of terrorists calmly pushing luggage carts in an airport, it is not their courage that stands out, nor their willingness to die, but the sense that death does not mean to them what it means to us. We speak of dehumanizing people by regarding them as “other” or alien. These terrorists are “other.” They are not like us in the fundamental sense that they say they prefer death over life—and by every indication, they do. We are, of course, terrified by the randomness and the violence of the terrorists, but what is more frightening is the terrorist himself.Recognition of the problem is at least a first step to dealing with it. Treating it as a disease may be the right approach. You isolate the disease, get rid of it, and then vaccinate the rest of the population against it coming back. I know, terrorism is complicated, but so are most diseases.