Rare Diseases Epidemiology is a novel action field still largely unexplored. However, Rare Diseases is a topic of growing interest at world level. The aims of this chapter are to revise useful epidemiological tools and define areas where epidemiology can help improve the rare disease knowledge, and facilitate policy decisions taking into account the real burden of rare diseases in society. This chapter also seeks to describe: the problems of coding and classification of diseases, measuring disease frequency, the study designs and association studies, the causality, the evolution from descriptive to epigenetic epidemiology and the natural history of disease. One of the major challenges facing analytical epidemiology and clinical epidemiological research into rare diseases is that genes can be involved in both aetiology and prognosis. Despite the many similarities between genetic association studies and classic observational epidemiological studies, the former pose several specific limitations, including an unprecedented volume of new data and the likelihood of very small individual effects, as well other limitations. Selecting the appropriate pathway from among all those available, i.e. the one that best relates genes from the various known regions and disease mechanisms, is crucial for the success of this type of studies.

Neonatal mortality during the first week of life, corresponding to the years 1975-1998, was studied in Spain. The first week of life is the time in which the highest number of deaths occur. The temporal decrease of the neonatal mortality rate (NMR) was modelled according to log10(NMR+1)= 2.784 - 0.023 per year. This decline cannot be explained by an increase in the mean birth weight (MBW=23440.835 - 10.107 g per year). From the most frequent of the causes of death to the least were: congenital anomalies, preterm born or low birth weight, respiratory problems, pregnancy difficulties, hypoxaemia/asphyxia, delivery difficulties and infectious diseases. This sequence changed when the specific age at death was considered. The NMR descended evenly for both sexes for the causes indicated above, except for preterm born or low birth weight, in which the male mortality decrease was greater since its rate was more elevated at the beginning of the period studied. For all the causes listed, NMR was more elevated both in urban areas and for males. Early neonatal mortality (first 24 hours) was higher for pregnancy difficulties, preterm born or low birth weight, congenital anomalies and hypoxaemia/asphyxia.

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