https://www.faculty.umb.edu/peter_taylor/epi/oxford05.pdf
The conclusion of that article is that the basic lessons we learn today, were - in part - learned in 1918...
Back then, researchers had already traced the emergence of outbreaks to an encampment in France where men and animals were living in close quarters and unsanitary conditions.
As to the first part of your question, the answer is that more people encroaching on the animal and wildlife territory while not adhering to basic sanitary practices will increase the chance of new strains appearing and making the jump to humans.
Vox did an excellent video on how this works for Corona: https://www.youtube.com/watch?v=TPpoJGYlW54
Such events do not only happen in China in particular but they can happen virtually anywhere in the world.
Another ongoing pandemic is HIV/AIDS. Well, it is generally accepted that the jump to humans occurred in West-Africa in the 1920's. But it took time and globalization - increasing mobility - before HIV finally spread across the world: