1. Department of Social Medicine and Public Health, Faculty of Public Health, Medical University, Plovdiv, Bulgaria
2. Department of Social Medicine and Public Health, Faculty of Public Health, Medical University, Bg - 4000 Plovdiv, Bulgaria
3. Department of Epidemiology and Disaster Medicine, Faculty of Public Health, Medical University, Plovdiv, Bulgaria
4. Department of Pediatric Dentistry, Faculty of Dental Medicine, Medical University, Plovdiv, Bulgaria