Examination of a 22-year-old man suffering from polyarthralgia and high fever revealed right-sided exudative pleuritis. X-ray picture showed a homogenous shadow below the IV rib on the right. In the II segment there were single dense focal shadows. Mantoux test with 2 TU resulted in formation of a papula 16 mm large. Pleural liquid has increased protein concentration, Ri-valta's reaction is positive, there was also increased number of leukocytes with prevailing lymphocytes. What is the most probable etiology of pleuritis?