Original: The III-S strain covers itself with a polysaccharide capsule that protects it from the host's immune system, resulting in the death of the host, while the II-R strain doesn't have that protective capsule and is defeated by the host's immune system.
Simplified: The III-S strain covers itself with a polysaccharide capsule that protects it from the host's immune system. The II-R strain does not have that protective shield around it and is killed by the host's immune system.