The ordinary lunatic is generally a harmless, isolated case; since everyone sees that something is wrong with him, he is quickly taken care of. But the unconscious infections of groups of so-called normal people are more subtle and far more dangerous.
The quote highlights the distinction between an individual's insanity and the dangerous tendencies of group behavior. While a person with an obvious mental health issue is often readily identified and managed due to their clear symptoms, the behaviors of supposedly normal individuals can be insidious. These "normal" people may perpetuate harmful ideologies or actions without realizing it, creating a more significant risk to society.
C.G. Jung suggests that the collective mindset of groups can lead to unconscious patterns of thought and behavior that pose greater threats than an isolated person’s madness. This underscores the importance of being aware of societal influences, as they can lead to a dangerous normalization of harmful ideas that go unnoticed until it is too late.