NEW ROCHELLE, N.Y. — The restaurants on North Avenue were nearly empty. The stores were out of spray disinfectant, rubbing alcohol and bleach. At a convenience store, the shopkeeper finished a transaction with a squirt of hand sanitizer, first for himself and then his customer.

This was life in the latest ground zero for coronavirus in America, a small Westchester County city about five miles north of New York City, as it grappled with the revelation on Tuesday that one of its residents — a lawyer who works in Manhattan — had contracted the virus and may have exposed hundreds of others to it.

By Wednesday, it was clear that the illness had spread: Gov. Andrew M. Cuomo announced at a morning news conference that the man’s wife, 20-year-old son and 14-year-old daughter all tested positive for the virus, as did a neighbor who drove him to the hospital last week. By day’s end, five more positive cases were disclosed: a man who was in “close proximity” with the Manhattan lawyer, as well as his wife and their three children.