The woman, who was not identified, was admitted April 13 with a fever, swollen lymph nodes and other symptoms. A blood test confirmed she had contracted the bacterial disease, county health officials announced Tuesday.
The woman was placed on antibiotics and is in stable condition, officials said.
Bubonic plague is not contagious, but if left untreated it can morph into pneumonic plague, which can be spread from person to person. Bubonic plague is usually transmitted to humans from the bites of fleas infected by dead rodents.
Health officials suspect the woman was exposed to fleas in her central Los Angeles home, said Dr. Jonathan Fielding, the county's director of public health. The woman's family was also placed on antibiotics as a precaution, but there is no evidence they were infected.
The case is unusual because it occurred in an urban area, Fielding said. Most bubonic plague outbreaks happen in rural communities.
Health officials said there was no cause for panic because the disease is not easily transmissible.
"There's no cause for alarm in the community," Fielding said.
Health officials went to the woman's home Tuesday to trap squirrels and other wild animals. Blood samples from the animals will be sent to a lab to determine if any are infected.
An estimated 10 to 20 Americans contract plague each year, mostly in rural communities. About one in seven cases is fatal, according to federal statistics.
The last human cases of plague in Los Angeles County occurred in 1984 when three people contracted the disease. Two of those cases were travel-related and the third involved a person exposed to a sick animal. All three survived.
Bubonic plague, also known as the Black Death, killed an estimated 25 million people in Europe between 1346 and 1351. The last modern outbreak in the United States occurred in Los Angeles in 1924-25, when at least 30 people died.
The plague is considered a bioterrorism agent, and state law requires doctors to report suspected cases to local health departments.