As a viral video ricocheted around China’s internet, showing toddlers and infants in a Shanghai medical facility, crying as they were crammed in threes and fours on metal-barred beds, one 39-year-old mother was particularly distraught.

“I searched for any sight of my daughter in the video but couldn’t find her,” said the mother, who asked to be identified only by her surname, Zhu.

Ms. Zhu said she had been separated from her 2½-year-old daughter on Tuesday after they tested positive for Covid in Shanghai, home of what is quickly becoming China’s biggest coronavirus outbreak in more than two years.

In the following days, Ms. Zhu said she was left with virtually no news or photos of her daughter, who she said was sent to a separate facility for Covid-infected children and infants.