1. Politely inform them:
“Unfortunately, I might not be able to clean your home if somebody in your home is sick or has been exposed to COVID”
2. If, they are currently experiencing COVID related symptoms, say:
“I am so sorry that you are feeling bad and I hope you start feeling better soon. At this time, I will not be able to clean your house, but someone from the office will contact you to reschedule the appointment when you feel better.”