There are a multitude of reasons why there are more diseases in more impoverished nations in the world.
1. Lack of access to clean water
People being forced to drink dirty water can lead to diseases. Some may include cholera, and other water-based bacteria diseases.
2. Lack of proper medical care
Without proper healthcare, people can easily go untreated for many diseases that can later be spread amongst the population.
3. Lack of proper sanitation
Tying back into lack of clean water, you won't be able to properly disinfect yourself. This can lead to unhealthy amounts of bad bacteria accumulating on your body and eventually getting inside you.