The conditions which have chiefly fostered it are the immigration of people who are accustomed to community life, the increase in factory life and the increased number of people of wealth who seek the advantages which the city gives them.

The city has always been the favored playground for the social game.

The unhealthy conditions of city life are due to the crowding, the more uncertain means of livelihood, the greater influence of vice and alcoholism.
Prostitution and the sexual diseases are almost the prerogatives of the cities.
3.

All means of transportation have increased and communication between peoples has become more extended and more rapid.

In the past isolation was one of the safeguards of the people against disease.
With the increase and greater rapidity of communication there is a tendency not only to loss of individuality in nations as expressed in dress, customs, traditions and beliefs, but many diseases are no longer so strictly local as formerly--pellagra, for example.