Sample size: In order to achieve an accuracy of 0.4% in the estimate of a percentage through a two-tailed 95% confidence interval, assuming the prevalences indicated by age strata in the general Spanish population (0.4%, 1.3% and 3.2% respectively), a total of 16,285 subjects distributed as follows: 1,303 aged 20-34 yr; 4397 aged 35-49 yr and 10,585 aged 50-79 yr.