Final answer:
Typhoid Mary was returned to North Brotherly Island because she continued to work as a cook and spread typhoid fever.
Step-by-step explanation:
Typhoid Mary was ultimately returned to North Brotherly Island because she continued to work as a cook and spread typhoid fever, despite being quarantined previously and being banned from working with food. Mary Mallon, also known as Typhoid Mary, was an asymptomatic carrier of the typhoid bacteria, meaning that she carried the bacteria and could spread it to others without showing any symptoms herself.