Scaling Health adds a nice challenge to the world. Mobs health will scale with your local difficulty. The longer you play, the more the difficulty scales up. The farther from your world spawn, the harder the mobs. 

Everytime you kill a mob your difficulty increases slightly. This includes boss mobs but at a slightly higher rate. You may come across mobs with purple flames. 
These are called Blight mobs and are much higher in difficulty than regular mobs. You should not engage these mobs unless well geared. 

Difficulty will also be altered temporarily by lunar phases. When the moon is full, mobs may hit a little harder and have more health than on nights without a full moon. 

Mobs will have a chance to drop heart canisters on death. Consume these to increase your own health. Mobs killed by fake players, i.e. mob farms, will not drop heart canisters. 
However, fake player mob kills will increase the local difficulty. 
