No matter what country in the world you visit, there is a perennial problem when visiting big cities. Any city that is synonymous with tourism also has another unfortunate association; a criminal association. As cities are prime preying ground for the less scrupulous of society, tourists are among the most likely people to be a victim of crime - and particularly, theft.
The problem is not specific to any one country, nor any one city. It is just, unfortunately, something that goes with the territory. Tourists are a target for criminals because tourists are out of their comfort zone; away from their home, perhaps not aware of the way things are done in a different country, and - most importantly - more likely than regular citizens to be carrying their valuables with them.
Tourists are a target for theft based crimes; theft of both property - such as wallets, money and jewelery - and also identity theft, such as passports being stolen. The statistics more than double when you look at the plight of tourists in big cities such as London, New York and Paris. Thieves rely on tourists to be disorientated and unfamiliar with their whereabouts, and the crowded locations make pick-pocketing an easy day's work for many a criminal.
While you should not let the possibility of crime ruin your holiday, so allow it to take part in your holiday preparations. Try and book hotels that are well-recommended on travel review websites, and preferably that have safes in each room. Consider a money belt to conceal cash and credit cards while out, and wherever possibly, don't take your passport out in to a city you are visiting.