[1] Dept. of Epidemiology,
[2] National Center of Infectious and Parasitic Diseases,undefined
[3] 26 Yanko Sakazov Blvd.,undefined
[4] 1504 Sofia,undefined
[5] Bulgaria; Phone: (+35/92) 46-5517,undefined
[6] Fax: -5517,undefined
[7] e-mail: epidep@ttm.bg,undefined
[8] Dept. of Virology,undefined
[9] National Center of Infectious and Parasitic Diseases,undefined
[10] Sofia,undefined
[11] Bulgaria,undefined
[12] Dept. of Epidemiology,undefined
[13] Ministry of Health,undefined
[14] Sofia,undefined
[15] Bulgaria,undefined
[16] Dept. of Epidemiological Control,undefined
[17] Hygiene-Epidemiological Inspectorate,undefined
[18] Sofia,undefined
[19] Bulgaria,undefined
[20] Dept. of Epidemiological Control,undefined
[21] Hygiene-Epidemiological Inspectorate,undefined
[22] Plovdiv,undefined
[23] Bulgaria,undefined
[24] Dept. of Epidemiological Control,undefined
[25] Hygiene-Epidemiological Inspectorate,undefined
[26] Varna,undefined
[27] Bulgaria,undefined