One of the most important themes of Sense and Sensibilityis, of course sensibility, and in many ways Austen's novel is critical of romanticism, especially in regard to Marianne Dashwood's overly passionate nature. In this way, sickness also becomes a minor theme of the novel, as Marianne's illness at Cleveland reveals the dangers of exposing oneself to potential heartbreak and disappointment.
Marianne's sickness at the Palmer's house is undoubtedly portrayed as a true illness, not just melodramatics of a jilted lover; Austen includes details to make sure the reader understands that Marianne suffers physically, like the description of her fever and chills. The important aspect about Marianne's sickness is how Austen uses it to reveal how an overly passionate nature can be a detriment to both the body and soul. Marianne's over-the-top romanticism led her to make poor choices for herself, not just in trusting her heart to Willoughby, but also in her penchant for mournfully wandering about the damp, rainy gardens. Austen utilizes sickness as a theme in Sense and Sensibility to warn readers against the dangers of allowing one's passionate nature to overshadow good sense and rationality.