def get_infection_probas_mean_field(probas, transmissions):
    """
    - probas[i,s] = P_s^i(t)
    - transmissions = csr sparse matrix of i, j, lambda_ij(t)
    - infection_probas[i]  = sum_j lambda_ij P_I^j(t)
    """
    infection_probas = transmissions.dot(probas[:, 1])
    return infection_probas