Frank King has gotten death threats. So many angry strangers have called his phone in recent weeks that he changed his number. And his neighbors have offered to bring food to his doorstep if he would just agree to stay inside his house.

“I have a whole new respect for the plight of pariahs,” said Mr. King, who was among 650 Americans who returned to the United States last month after being stuck for more than a week on a cruise ship that no country initially allowed to dock because of fear of coronavirus.

One person from that ship, the Westerdam, was said to have briefly tested positive on the way home, sparking an international panic. In the days that followed, 1,580 crew members and passengers tested negative, including Mr. King. Nonetheless, Mr. King has been the target of rage because of his decision to fly home before he received the test results.

The scathing criticism he has received illustrates how swiftly societal pressure can bear down on people who are perceived to be endangering public health, even if they pose no actual threat.