Parts of the Angeles National Forest remain closed this week after a squirrel caught there tested positive for bubonic plague.
This potentially deadly strain of bacteria has been in California for more than 100 years, dating back to 1898, according to Northern Arizona University micro-biology professor Paul Keim.
The bacteria likely came on cargo ships from China, traveling on infected rats to the port of San Francisco.
"They got into the city and started spreading out," Keim said. "And before you knew it you had a disease of plague in the rats of San Francisco, then via the fleas they would jump over to people."
The outbreak lasted almost a decade, killing close to 200 people. From there, the bacteria likely spread through rats, ground squirrels and other rodents eventually reaching Los Angeles in 1924.
The city set up a quarantine to control spread of the disease and launched a rat extermination program.
"There just weren't antibiotics," said Keim.
37 people died. It was the last urban outbreak of the disease in the United States.
Now, bubonic plague is rarely found in humans and treatable when identified. There have been just four cases in Los Angeles county since 1984, none were fatal.
But it's still very common in wild rodents. The Center for Disease Control recommends using insect repellent when camping to keep away fleas and making sure pets don't run wild in areas where sick rodents could be found.