Ok…what were the death rates. Do you know? Was it really that bad?
NYC would have rapidly died out had it not been sucking in people from the outside constantly because the death rates were vastly higher than the birth rates. One of the big infrastructure projects paid for by federal money raised by tariffs overwhelmingly on Southern goods was the NYC Sewer system. It took quite a while to construct it and get all the sewage out of town so there wouldn't be quite as much disease. For example, NYC had big cholera outbreaks in 1832, 1849 and 1866. The story in other big cities in the North wasn't that different at the time.
It was very bad.