When is a bus driver not allowed to drive?

You answered correctly. According to German traffic rules, a bus driver is not allowed to drive in three specific situations to protect public health and safety. First, if the driver is suffering from a notifiable contagious disease, they must not drive to prevent spreading the illness to passengers and others. Second, if a member of the driver's household has such a disease, the driver is also prohibited from driving, as there is a risk of infection. Third, if the driver is seriously indisposed, meaning physically or mentally unfit to drive, they must not operate the bus to ensure road safety. These rules help minimize health risks and ensure that bus drivers are capable of safely transporting passengers.