1. A method of constructing a classifier for disease detection, adapted to an electronic apparatus, comprising: constructing a codebook of representative features based on a plurality of disease-irrelevant data; extracting a plurality of transfer-learned disease features from a plurality of disease-relevant bio-signals according to the codebook, wherein both the disease-irrelevant data and the disease-relevant bio-signals are time-series data; and performing supervised learning based on the transfer-learned disease features to train the classifier for disease detection.