Answer: Most likely the industrial revolution.
Explanation: The use of machines in production became more and more prevalent, eventually many factories were built in cities which caused pollution therefore making the air gross.
It was Called the frontier because some or most of the areas in America that where settled by ranchers hadn't been explored or discovered yet.
The Black Death begin in October 1347 in Europe. Opps. I didn't read the question correctly.
The Black Death was a horrific plague. It made people feel ill and gave people nasty black boils. The boils oozed out blood, hence the name, "Black Death". The total death were about 20 million.
