Growing up in the United States Virgin Islands, Dr. Marcella Nunez-Smith saw firsthand what can happen in a community with limited access to health care. Her father, Moleto “Bishop” Smith Sr., was only in his 40s when he suffered a debilitating stroke that left him partly paralyzed and with slurred speech.

The cause was high blood pressure, which could have been treated but had never been diagnosed. Without prompt access to advanced treatments, “the stroke was allowed to run its course,” Dr. Nunez-Smith, 45, recalled in a recent interview. Her father never fully recovered.

DURBAN, South Africa — Doctors and nurses at a South African hospital group noticed an odd spike in the number of Covid-19 patients in their wards in late October. The government had slackened its lockdown grip, and springtime had brought more parties. But the numbers were growing too quickly to easily explain, prompting a distressing question.

“Is this a different strain?” one hospital official asked in a group email in early November, raising the possibility that the virus had developed a dangerous mutation.