Religious schools have been shut.

Some festivities for the Jewish holiday of Purim are up in the air.

And officials in Westchester County, N.Y., estimate 1,000 people are quarantined at home after a well-attended bat mitzvah and funeral.

The case of a seriously ill father with deep roots in a modern Orthodox community shows how quickly coronavirus can spread in circles that live, go to school and attend services together. As of Thursday, officials said 18 people in Westchester had been diagnosed with Covid-19, including the man to whom these cases are connected.

Rabbis said the connectedness of their community is a great strength, but when a contagious illness strikes, the vast network of links among families can make it especially hard to control infection.