He had built a wall around him that was preventing his normal interaction with people. This was causing real suffering and sickness. "What then became of me? I know not; I lost sensation, and chains and darkness were the only objects that pressed upon me." (p.168) He loses interest in life even more when his dear ones are killed: "I had formed in my own heart a resolution to pursue my destroyer to death; and this purpose quieted my agony, and provisionally reconciled me to life." (p.169) Sickness is thus a multifaceted theme in the novel. It serves many purposes. On the one hand, we see it as a force fighting against the evil ambitions of Victor and on the other, it can also be seen as a compassionate force trying to restrain Victor. It is all a matter of perception. Had Frankenstein understood why he was falling...

When it wasn't physical, it was purely emotional or mental. His weak nerves came under repeated attacks of melancholia and gloom. Sickness is thus an important theme in the novel that highlights some key features of the novel.
Works Cited

Anne K. Mellor, " Making a "monster": an Introduction to Frankenstein," the Cambridge Companion to Mary Shelley, ed. Esther Schor (Cambridge, England: Cambridge University Press, 2003)

The Mary Shelley Reader, eds. Betty T. Bennett and Charles E. Robinson (New York: Oxford University Press, 1990)