The Flaw

Although the immune system is very good at protecting the body from diseases, it does have one major flaw: It can only confer immunity upon the body from illnesses that it survives. Hence, if someone does not survive an illness, then they do not have immunity towards it, and die.

Hence, it is important to remember that pathogen-saturated environments may expose the body to pathogen attacks that it cannot recover from, and hence, pathogen-saturated environments are not without their ill effects.