Mrs. Warren's family came down with Typhoid fever shortly after hiring their new cook, Mary Mallon.  It was soon discovered that Mary was a carrier of typhoid even though she appeared to be healthy.  In the 1900's, people didn't understand germs as well as we do now.  It was difficult to make Mary understand that she was making others sick, so they ended up putting her in quarantine for a few years to study her.  When she was released, she was warned that she could no longer be a cook, but she didn't follow the rules and was sent back to quarantine.

Website: https://www.drinkingthecoolaid.com/  

Resources:
w25689.pdf (nber.org)

Terrible Typhoid Mary A True Story Of The Deadliest Cook In America by Susan Campbell Bartoletti