**2. Epidemiology**

The prevalence of COPD varies greatly per country and also within countries [9]. This heterogeneity can be contributed to not only differences in diagnostic methods and classifica‐ tion but also to smoking habits, population age, in- and outdoor air pollution, occupational exposure, prevalence of pulmonary tuberculosis, chronic asthma and socioeconomic status [10]. Prevalences of COPD have been reported varying from 0,2-37% [11, 12]. The prevalence of AECOPD is very difficult to determine since there is no generally agreed definition for an AECOPD (see above). Studies show that only 32-50% of symptom defined AECOPD are reported by patients to health care professionals [13, 14]. Although there is no reliable estimate of the prevalence of AECOPD, much is known about the occurrence of exacerbations. Research shows that exacerbations are more frequent in the winter season [15] and may occur clustered in time [16]. Exacerbations are also more frequent and severe as COPD severity increases [17]. Besides COPD severity, the history of exacerbations is also a good predictor of future exacer‐ bations [17]. Furthermore, there is a strong correlation with symptoms of depression and recurrent exacerbations [18, 19].
