## 1 Introduction of Periodontitis  
### Overview of Periodontitis and Its Prevalence  
Periodontitis is an inflammatory condition of the periodontal tissues characterized by the progressive destruction of the tooth-supporting structures. The primary features include the loss of periodontal tissue support, which is evidenced by clinical attachment loss (CAL) and radiographically assessed alveolar bone loss, as well as the presence of periodontal polyceting and gingival bleeding. Periodontitis is a major public health problem due to its high prevalence, as well as because it may lead to tooth loss and disease, negatively affect chewing function and aesthetics, be a source of social inequality, and impair quality of life [1]. According to the latest international classification scheme, periodontitis is further subdivided as follows: periodontal health and gingival diseases and conditions; periodontitis; periodontal manifestations of systemic diseases and acquired conditions; and peri-implant diseases and conditions [2]. The classification of periodontitis according to the EFP (European Federation of Periodontology) includes four stages (I to IV) and three grades (A to C), which assess the severity, complexity, and rate of disease progression. The grades (A, B, C) reflect the risk of progression, influenced by factors such as smoking and diabetes. The stages (I-IV) classify the extent of periodontal tissue loss, including clinical attachment loss and bone loss [3]. These are multifactorial conditions primarily driven by bacterial etiologies. The development and progression of these diseases are influenced by three key factors [4]:  
1. Host Susceptibility: Genetic and immune system factors that predispose individuals to periodontal disease;
2. Environmental Factors: External influences, such as environmental pollutants, diet, and microbiome composition, that may exacerbate or mitigate the progression of periodontal disease;
3. Behavioral Factors: Lifestyle choices, including oral hygiene practices, smoking, and dietary habits, which significantly impact the onset and severity of periodontal conditions.  
A comprehensive understanding of the complex interactions between these cofactors is essential for developing effective strategies for the prevention, diagnosis, and management of both periodontal and systemic health conditions.  
Periodontitis is the most common chronic inflammatory non-communicable disease in humans. According to the Global Burden of Disease 2010 study, the global age-standardized prevalence of severe periodontitis (1990-2010) was 11.2%, making it the sixth-most prevalent condition worldwide. In the Global Burden of Disease 2015 study, the prevalence of severe periodontitis was estimated at 7.4%. Periodontal diseases are a major global public health issue, with prevalence rates reaching up to 50% worldwide [5].  
Patients with periodontal diseases are at increased risk of tooth loss, dentulism, and masticatory dysfunction, all of which negatively impact their nutrition, quality of life, and self-esteem. Importantly, periodontitis is a chronic condition that cannot be cured but can only be managed through long-term treatment and maintenance therapy [6]. Furthermore, periodontal infections are associated with a range of systemic diseases, contributing to premature mortality. These include diabetes mellitus, cardiovascular diseases, rheumatoid arthritis, Alzheimer's disease, and adverse pregnancy outcomes [7].