531,834 PROBLEMS Problems of infectious and parasitic diseases / [ed.-in-chief B. Petrunov]. - Sofia s National Center of Infectious and Parasitic Diseases. - 29 cm Vol.19. - 1992. - 61 s. : 11., wykr. Vol.20. - 1993. - 59 s. : 11., wykr. Vol.21. - 1994. - 45 s. : 11., wykr.