MONROVIA, Liberia — The girl in the pink shirt lay motionless on a sidewalk, flat on her stomach, an orange drink next to her, unfinished. People gathered on the other side of the street, careful to keep their distance.

Dr. Mosoka Fallah waded in. Details about the girl spilled out of the crowd in a dizzying torrent, gaining urgency with the siren of an approaching ambulance. The girl’s mother had died, almost certainly of Ebola. So had three other relatives. The girl herself was sick. The girl’s aunt, unable to get help, had left her on the sidewalk in despair. Other family members may have been infected. Still others had fled across this city.

Dr. Fallah, 44, calmly instructed leaders of the neighborhood — known as Capitol Hill, previously untouched by Ebola — how to deal with the family and protect their community. He promised to return later that day, and send more help in the morning. His words quelled the crowd, for the moment.

“This is a horrific case,” he said as he walked away. “It could be the start of a big one right here. It’s a ticking time bomb.”