Complex sentence: Microbial symbiosis and immunity refers to the close and often long-term relationships between microbes and their host's immune system, which can be commensal, mutualistic, or parasitic.
Split into simpler sentences: Microbial symbiosis and immunity refers to the close and often long-term relationships between microbes and their host's immune system. These relationships can be commensal, mutualistic, or parasitic.