It’s not known why one person has a mild form of psoriasis and another a severe form or why one person has nail changes while another does not. Over time, psoriasis severity and location may change and evolve or get better or worse.
Unfortunately, more is known about what makes psoriasis get worse than what makes it get better. Strangely, the severity of psoriatic arthritis and the severity of skin psoriasis often do not correspond.
Research thus far shows that the degree of treatment, whether very aggressive or none at all, does not alter the course or severity of the disease. Early onset of psoriasis, in the early 20s or before, is more strongly linked to a family history and is more likely to become severe over time.
In the large study at Stanford University described in Question 26, almost 40% of people experienced disease-free periods of over 1 year. These statistics may have improved in the subsequent 30 years because of the many new therapies developed since then.