How many Muslims will this ISIS virus be able to infect in the vast European "gray zone"? The answer will determine our future.

Even if these polls and surveys must be taken with some caution, they all indicate a deep and vibrant "gray zone," which is feeding the Islamic jihad in Europe and the Middle East. We are talking about millions of Muslims who show sympathy, understanding and affinity with the ideology and goals of ISIS.

Among young European Muslims, support for suicide bombings range from 22% in Germany to 29% in Spain, 35% in Britain and 42% in France, according to a Pew poll. In the UK, one in five Muslims have sympathy for the Caliphate. Today more British Muslims join ISIS than the British army. In the Netherlands, a survey shows that the 80% of Dutch Turks see "nothing wrong" in ISIS.

In the 1970s and '80s, Europe was terrorized by a war declared by Communist armed groups, such as the Germany's Baader Meinhof or Italy's Red Brigades. Terrorists seemed determined to undermine democracy and capitalism. They targeted dozens of journalists, public officials, professors, economists and politicians, and in Italy in 1978, even kidnapped and executed Italy's former prime minister, Aldo Moro.

The big question then was: "How deep is the 'gray zone'?" -- the sympathizers of terrorism in the industrial factories, labor unions and universities.

In the last year, the Islamic State's henchmen slaughtered hundreds of Europeans and Westerners. Their last assault, in Brussels, struck at the heart of the West: the postmodern mecca of NATO and the European Union.

We should now answer the same question: How deep is the "gray zone" of the Islamic State in Europe?

Peggy Noonan recently tried to give an answer in the Wall Street Journal:

"There are said to be 1.6 billion Muslims in the world. ... Let's say only 10% of the 1.6 billion harbor feelings of grievance toward 'the West', or desire to expunge the infidel, or hope to re-establish the caliphate. That 10% is 160 million people. Let's say of that group only 10% would be inclined toward jihad. That's 16 million. Assume that of that group only 10% really means it — would really become jihadis or give them aid and sustenance. That's 1.6 million."

That is a lot.

According to a ComRes report commissioned by the BBC, 27% of British Muslims have sympathy for the terrorists who attacked the Charlie Hebdo office in Paris (12 killed). An ICM poll, released by Newsweek, revealed that 16% of French Muslims support ISIS. The number rises to 27% percent for those aged 18-24. In dozens of French schools, the "minute of silence" to commemorate the murdered Charlie Hebdo's journalists was interrupted by Muslim pupils who protested it.

How deep is ISIS's popularity in Belgium? Very deep. The most accurate study is a report from Voices From the Blogs, which highlights the high degree of pro-ISIS sympathy in Belgium. The report monitored and analyzed more than two million Arabic messages around the world via Twitter, Facebook and blogs regarding ISIS's actions in the Middle East.

The most enthusiastic comments about ISIS come from Qatar at 47%; then Pakistan, at 35%; third overall is Belgium, where 31% of tweets in Arabic on the Islamic State are positive -- more than Libya (24%), Oman (25%), Jordan (19%), Saudi Arabia (20%) and Iraq (20%). This shocking data exposes the success of the network and its easy pro-ISIS recruitment in Belgium.

In other European countries, after Belgium, Britain is at 24%, Spain 21%, France 20%.

In the UK, one in five Muslims have sympathy for the Caliphate. Today more British Muslims join ISIS than the British army.

In the Netherlands, a survey conducted by Motivaction shows that the 80% of Dutch Turks see "nothing wrong" in ISIS.

Among young European Muslims, support for suicide bombings range from 22% in Germany to 29% in Spain, 35% in Britain and 42% in France, according to a Pew poll.

The level of ISIS's popularity in the Arab world has been exposed by many surveys: the Clarion Project published a report based on multiple sources a March 2015 poll by the Iraqi Independent Institute for Administration and Civil Society Studies, a November 2014 poll by Zogby Research Services, a November 2014 poll by the Arab Center for Research and Policy Studies, and an October 2014 poll by the Fikra Forum. The result: 42 million people in the Arab world sympathize with ISIS.

After the massacre at Charlie Hebdo, Al-Jazeera conducted a survey asking, "Do you support Isis's victories?" 81% of respondents voted "yes."

Even if these polls and surveys must be taken with some caution, they all indicate a deep and vibrant "gray zone," which is feeding the Islamic jihad in Europe and the Middle East. We are talking about millions of Muslims who show sympathy, understanding and affinity with the ideology and goals of ISIS.

Anthony Glees, an English scholar of political radicalism, revealed the "gray zone" of Germany's Baader-Meinhof terror group: "By 1977, the West German Federal Criminal Agency had a terrorist index which contained the names of some 4.7 million suspects and sympathisers, many of them university students."

The terrorist leaders at that time all came from good German families: Andreas Baader was the son of a professor of history, Ulrike Meinhof was the daughter of a museum director and a famous journalist, Gudrun Ensslin was the daughter of an evangelical pastor, Horst Mahler was the son of a judge.

The Islamic State today has a much deeper gray zone of sympathizers in the Muslim communities of Europe.

If Baader-Meinhof was at war with the "schweine" (bourgeois "pigs") and targeted specific political figures, the Caliphate's volunteers are at war with all the "kuffar" (unbelievers). ISIS loyalists target the patrons of restaurants, theaters and stadiums in Paris; a café in Copenhagen which held a debate on freedom of expression and Islam; Western tourists at a resort in Tunisia; commuters at the Maelbeek metro station and passengers at the Brussels airport.

For ISIS, it is an eternal war in the name of the prophet. As Graeme Wood explained in "What ISIS Really Wants," ISIS "hungers for genocide ... and it considers itself a harbinger of — and headline player in — the imminent end of the world."

A book just published in French by Ivan Rioufol, a journalist for the newspaper Le Figaro, eloquently titled "The Coming Civil War," details the dangers posed by the "apocalyptic ideology" of radical Islam in Europe. How many Muslims will this ISIS virus be able to infect in the vast European "gray zone"? The answer will determine our future.