Epidemiology is the study of health phenomena that affect communities or human groups and the factors that influence these patterns, depending on time, place and people. It tries to determine, over the years, whether a certain phenomenon or characteristic (for example, illness) has increased or decreased, if its frequency in one geographical area is greater than in another, and if people who have that certain phenomenon show characteristics different from those that do not. General practice is an important source of information on the occurrence and distribution of disease in the community 

Epidemiology is the study of health phenomena that affect communities or human groups and the factors that influence these patterns, depending on time, place and people. It tries to determine, over the years, whether a certain phenomenon or characteristic (for example, illness) has increased or decreased, if its frequency in one geographical area is greater than in another, and if people who have that certain phenomenon show characteristics different from those that do not. General practice is an important source of information on the occurrence and distribution of disease in the community [1]. For most illnesses general practitioner (GP) is the first point of contact in the health care system and he looks after a population whose age and sex composition are known. He/she is therefore in an ideal position to conduct inquiries about natural history of disease (the factors predisposing, precipitating and perpetuating the disease) [2].
The doctor-patient relationship has been described as “a consensual relationship in which the patient knowingly seeks the physician’s assistance and in which the physician knowingly accepts the person as a patient." [3]. Doctor-patient relationship is a complex phenomenon made up of several aspects, including doctor-patient communication, patient participation in decision-making and patient satisfaction. Doctor-patient relationship has changed throughout time, and is currently being redefined so that both the doctor and patient have a role in treatment decisions [4]. The importance of studying this relationship is given by the fact that its influence on health care outcomes has been established for years [5-8]. In the field of primary care it has been observed that certain styles of communication that are more empathetic, more patient-centered and that offer security and support, are associated with better health outcomes [9, 10].
Relationships are linked to emotions and emotions have a physiological basis. The distressing emotions of fear and pain that so often accompany patients' symptoms drive them to seek relief through medical care, an important ingredient of which is the affective care of the doctor [11]. Doctor-patient relationship models, depending on the interrelationship established between doctor and patient, imply different decision-making (diagnosis and treatment). This situation would give rise to different prevalences, incidences, sensitivities, specificities, predictive values, etc., different depending on the type of doctor-patient relationship and therefore, ultimately this has "hidden" epidemiological implications in the morbidity data in primary care (prevalence and incidence of diseases and their distribution in the population). Consequently, there is no doubt that the doctor-patient relationship is an aspect of great importance in the treatment of patients, but also in epidemiological information [12]. Therefore, the study of the doctor-patient relationship is of special epidemilogical importance [13].
However, it is a poorly defined and difficult to measure topic (14). From both the patient and physician perspectives, numerous questionnaires have been used [15-18]. To comprehensively assess its epidemiological importance, a clearer understanding of the concept is required. Active components may be loyalty, personal attention, trust, continuity, empathy, knowledge, respect, confidentiality, informed consent, shared decision making, physician superiority/conversational dominance, conflict of interest, etc. Furthermore, doctor-patient relationship is a complex, multiple and heterogeneous relationship that can not be defined in a unique way or generalize into an only concept of relationship, but there are "many" doctor-patient relationships appropriate according to their contexts.
There are a number of models for developing doctor-patient relationship [19-22].
1. Active-Passive Model: patient participates very little in the relationship. Patient seeks information and technical assistance, and physician makes decisions that patient must accept. In this model,  physician actively treats patient, but patient is passive and has no control.
2. Guidance-Cooperation Model
Physician recommends a treatment and the patient cooperates. The imbalance between physician and patient is not as pronounced as in the active-passive model, but the patient is expected to comply with the "doctor's orders" with little or no objection. It appears to be the most frequent in current medical practice; 80% of GPs are said to "take the patient's opinion into account" [23]. Other authors admit that patient-centered encounters are slightly more common (55%) than physician-centered encounters (45%) [24]. Other authors have reported that between 60-80% of patients say that the physician “always” takes into account the patient's opinion; and approximately 30% of patients report that the physician does not take into account their opinions “almost never.” [23].
3. Mutual Participation Model or "patient-centered"
Physician and patient share the responsibility for making decisions and planning the course of treatment. Patient and physician respect each other's expectations, point of view, and values. Physician's role is to help patients determine what their goals are and how they can achieve them [25]. It seems that this model is rarely used in primary care.
In addition, it can be hypothesized that the doctor-patient relationship is different in different types of health problems [26, 27]. Thus, according to psychosocial aspects of diseases, the doctor-patient relationship can be different: In cardiovascular diseases, control of verbal and non-verbal expressions is required to transmit security; in arterial hypertension, the relationship can be focused on persuasion that seeks to guarantee therapeutic compliance; in asthma and COPD, it is about "ventilating" the patient's conflicts; in digestive diseases, the patient usually has feelings of dependency; in psychiatric diseases, the capacity for silent listening can be maximized; in endocrinological problems, detailed explanations at a cultural level of the pathophysiological mechanisms may be required; in hematological diseases, the mental stability of the patient may be taken into account during chemotherapy; in cancer, it is about maintaining clear and permanent communication with the patient; in AIDS, it is focused on knowledge of the diagnosis and confidentiality; in rheumatic diseases, the relationship is focused on being seen as an individual and not as a mere diagnosis, and on being believed with regard to pain and suffering. In dermatological diseases, priority is given to explanations about the probable cause of the disease and its duration; in neurological diseases, the patient must be aware of his or her diagnosis and the evolution of the disease, anticipating possible emotional reactions, and the doctor-patient relationship extends to the family environment [27]. Likewise, there may be differences in the doctor-patient relationship in situations of multimorbidity [28], or in the elderly patient [29], as well as when pharmacological treatments are involved [30], and at the first visit or subsequent visits. But despite all this, most physicians tend to have a predominant style of interaction [24].
In summary, doctor-patient relationship is a fundamental concept to be able to use the morbidity data of general medicine. However, there are few data regarding the frequency of relationship patterns. Furthermore, given that the complexity of the articulation of these processes and factors cannot be easily analyzed using reductionist or only quantitative strategies, where studies should take into account numerous confounding variables, it is therefore necessary to include qualitative perspectives.