Today, yellow fever is mostly gone from the U.S. It still plagues parts of Africa and South America, but there's a vaccine that can help prevent it. Back in the day, though, the only way to develop immunity to the virus was to survive it.
As a result, Olivarius explains, a social hierarchy developed in New Orleans around who was "acclimated" (people who had lived through yellow fever) and "unacclimated" (people who hadn't).
"If you're unacclimated, you basically languish in professional and social purgatory," says Olivarius, who is writing a book about how the disease shaped the city's social structure. "Bosses will not hire clerks and bookkeepers who are not expressly acclimated. Women will not marry men not described as acclimated. You can't live in certain neighborhoods, and people will not rent rooms unless you're acclimated. Certain social circles will exclude you. And so this creates this hierarchy where you have people who are actively seeking to get sick."
The tricky part, she says, is that there was no real physical way to tell whether someone was acclimated. So people had to find ways to demonstrate that they were. That often involved showcasing how deep their ties were to New Orleans. People who had grown up in the city were more likely to have survived a mild case of yellow fever as a child.
But the city's many European immigrants, who hadn't been around the virus before, were considered bigger liabilities. Olivarius says they often arrived in New Orleans with already compromised immune systems and lived in neighborhoods with no herd immunity. That's part of the reason, she says, that yellow fever was nicknamed the "Stranger's Disease."
Still, immigrants were flocking to the city. Olivarius says that New Orleans in the 19th century was a bit like Silicon Valley today: "It was the place where, if you were an ambitious white man, you went to make your fortune." People came from far and wide to try to break into the booming cotton industry, maybe eventually buy themselves some land and slaves. But first, they had to prove they weren't going to up and die.