Concerned about dog safety and uncomfortable with caring for a seriously ill dog, what should I do?

I accepted a booking for a dog knowing that he was sick and needed some medication. The owners didn't tell me what the illness was but simply said it was a stomach virus. Partway through the next day, Rover informed me that the dog had been diagnosed with canine parvovirus. I live in an apartment building with many dogs and also am frequently in contact with a young puppy. I am not comfortable caring for a dog who is contagious and has a life-threatening illness, especially without knowing this beforehand. What should I do?