A 32-year-old man comes to the clinic because of a 6-month history of intermittent itching and “flaky skin” on his scalp. He also notices greasy, yellowish scale and redness around his eyebrows and along the sides of his nose. He has tried changing shampoos without significant improvement. He is otherwise healthy and takes no medications. Physical examination shows erythematous plaques with greasy scale on the scalp and eyebrows. Which of the following best describes the underlying etiology of this condition?