Interaction of bacteria and immune system ~ Is bacteria good or bad?

There are many bacteria in our environment such as in soil, on food and in our body. Most of bacteria in our environment are harmless and important for our body. However, some of them are pathogenic and attack our body, although, we usually do not become diseased because of our immune system. The immune system is one of the mechanisms within our body that protect against disease by killing bacteria and virus. Therefore, it is important to understand how our immune system interacts with bacteria.