Final answer:
Illness anxiety disorder involves excessive worry about having a serious disease with little to no symptoms, while somatic symptom disorder involves distress from actual physical symptoms which may not have a clear medical cause.
Step-by-step explanation:
The difference between illness anxiety disorder and somatic symptom disorder lies primarily in the presence and intensity of the symptoms and the level of worry about the health implications of these symptoms. Illness anxiety disorder, previously known as hypochondriasis, is characterized by excessive concern about having a serious disease despite having few or no symptoms to support this belief. Individuals with this disorder are extremely anxious about the possibility of being unwell, even when there is no or minimal evidence to suggest a presence of a physical condition.
On the other hand, somatic symptom disorder is defined by one or more chronic physical symptoms that are distressing and may not have a diagnosable medical cause. The key feature of this disorder is that the individual's reaction to their symptoms, which could include high levels of anxiety, disproportionate concern about the seriousness of the symptoms, and excessive time and energy devoted to these symptoms, is considered excessive or out of proportion to the symptoms themselves.
Whereas illness anxiety disorder focuses on the anxiety about the potential of having a disease, somatic symptom disorder involves actual physical symptoms which may not be fully explained by a medical condition but nonetheless are very distressing and preoccupying for the individual.