How comes Doctors dont wear face masks when operating on animals but they have to when they operate on people?

';people'; diseases/germs are not transferrable to animals. I've worked with vets....some wear masks, and some don't ~How comes Doctors dont wear face masks when operating on animals but they have to when they operate on people?
They do wear face masks when operating on animals.








But the reason they have to is so they don't either spread germs to the patient or catch germs from the patient.