Summary: We present a two-stage group testing model for the detection of viruses in blood samples in the presence of random window periods. As usual, if a tested group is found to be positive, all its members are treated individually. The groups that were tested negative return for a second round after a certain time, and new blood samples are taken and tested after pooling. The system parameters are the size of the population to be screened, the incidence rates of the infections, the probability distributions of the lengths of the window periods, and the costs of group tests. The objective is to minimize the expected cost of running the system, which is composed of the cost of the conducted group tests and penalties on delayed test results and on misclassifications (noninfected persons declared to be positive and, more importantly, persons whose infections have not been identified). By an appropriate choice of the group size and the waiting time for the second round of testings one wants to optimize the various trade-offs involved. We derive in closed form all the probabilistic quantities occurring in the objective function and the constraints. Several numerical examples are given. The model is also extended to the case of several types of viruses with different window periods.