After Rachel Wall described her coughing spells, extreme fatigue and fevers, her doctors told her to self-quarantine in her Denver home for 10 days and until she was fever-free for 72 hours. In Cincinnati, Elizabeth Edwards was advised to stay home for 14 days and until she was symptom-free for three days.

In the Netherlands, Kevin Toms also got the 14-day directive. And in Australia, Amy McKenzie’s home isolation could last even longer: She needs two consecutive negative tests to show she isn’t shedding virus anymore—and...