A reusable piezoelectric immunosensor has been developed for the detection of viruses and bacteria, playing an important role in acute diarrhea in early childhood. Three different methods for antibody immobilization to the gold electrode were tested. Coating the electrode with protein A gave the best results in terms of stability, sensitivity and reproducibility. Worse results were observed when coating the electrode with γ-amino propyltriethoxy silane and polyethyleneimine. Applying an antibody layer via protein A immobilization onto a 10-MHz AT-cut crystal resulted for 1 × 106 to 1 × 1010 virus cells and for 1 × 106 to 1 × 108 bacteria in a linear frequency change. The modified crystal was stable for six weeks when stored desiccated at +4°C.