My doctor should wear something that has a filter capable of catching the virus, such as FFP3 respirator. When you breathe in, droplets get through the mask (this gets progressively worse as the mask gets wet) - and if the mask is not a FFP3, you're making a bet that none of the droplets that get through are contaminated, because the mask does not protect you from the virus, it merely protects you from some of the droplets around you; only the correct filter will protect you. The virus is around 50-100 nanometers.