Vascular tumors

A 25-year-old man comes to the office because of a growth in his right leg. Patient has HIV+ and patient says that at first, the lesion was flat, however, it has become raised over the past year. He says that an additional lesion has appeared in his right hip and another in his flank. Physical examination shows a large, raised, well-circumscribed, purple lesion in his right upper thigh. A biopsy of one of the lesions shows a lymphocytic infiltrate. Which of the following is the most likely etiology of this patient's condition?