I don't accept food or drinks from borrowers, ever. Use hand sanitizer immediately. Have sanitizer wipes to clean pens, your hands and anything the person has touched that can be wiped.
If you think the borrowers are ill when you confirm the appointment, ask them if they are ill. If you have a low tolerance to germs or a compromised immune system, don't take the signing.
Other than that, I can't think of anything else you can do.