If delivering supplies to people in coronavirus self-quarantine, should you avoid contact with the self-quarantined person's dog and cat too?

Probably no need: No human cases of COVID-19 have been reported due to contact with pets. There is a single report of a dog owned by an infected person testing positive. That one instance doesn't mean it's an important problem. I wouldn't worry about it. (3/8/20)

Low risk: There are rare case of cats & at least one dog testing positive for COVID 19. Theoretically someone who is COVID 19 positive could touch their animal & the virus could remain alive for a short period of time. COVID 19 doesn't like porous surfaces. Theoretically someone could touch the pet & pick up surface contamination. This is a low risk. No known cases of animals transmitting COVID to humans.