Some days it feels like we’re losing a battle against mosquitoes, ticks, fleas, flies and worms.
Just this month, tularemia, also known as rabbit fever and caused by a bacterium in ticks and deer flies, was detected in people living on Long Island. A park in Marin County was shut down in July after it was found to be teeming with fleas, which can carry plague and typhus. And in April, Rickettsia, a rare but deadly tick-borne bacterium, was found in a person living in Sonoma County.
Lyme disease and Alpha-gal syndrome are expanding their range. Dengue and typhus are increasing. There are still reports of West Nile, Zika and Chikungunya, plus the potential introductions of Chagas and rat lungworm disease. Some of these are contracted overseas, while others are seemingly settling in for the long haul.
Experts say there is no single reason for this surge in disease. Some of it may be hyped news, headline sensationalism. When everyone is writing about an outbreak of a disease, no matter how isolated, it can feel more pervasive and widespread than it actually is.
Increased monitoring and more advanced disease tracking can also amplify the sense that something new is afoot, even if it was always there.
But in some cases, there really does appear to be an increase — and California is front and center.
You have to ask, “what are we doing to the planet and does that mean we will encounter more diseases?” said Janet Foley, a disease ecologist at the UC Davis School of Veterinary Medicine. “The answer is: yes.”
This year, conditions including a record-smashing March heat wave led to a surge in California’s native Culex mosquitoes, the transmitters of West Nile virus. Most infections are asymptomatic, but about one in 100 involve neuroinvasive disease: meningitis, encephalitis or paralysis. There’s no specific treatment.
Matt Willis is a former Marin County public health officer who used to work at the Centers for Disease Control and Prevention as an epidemiologist and now helps model the health impacts of climate change. In a July 23 Substack post, he wrote that early signs point to a potentially severe season for West Nile virus in California. L.A. County’s positive mosquito samples were 10 times higher this year than last and nationally, human cases were at a 20-year high for that point in the season. The following week, he wrote that increased activity “has me even more concerned.”
As of mid-August, 373 West Nile virus-positive dead birds have been reported statewide, compared to 76 at the same point in 2025. The CDC confirmed 181 human cases nationwide — 129 of them neuroinvasive.
“As temperature rises globally, and as more northern latitudes experience hotter temperatures on average, and more extreme temperatures, some of those pathogens and/or insects that are more prevalent in those parts of the world migrate northward,” Willis said. “That is happening.”
Another mosquito-borne disease of increasing concern in California is dengue. Also known as “breakbone fever” — the painful, debilitating disease, which can be fatal, is caused by a virus adapted to an invasive, African mosquito known as Aedes aegypti.
A recent study in the Lancet Regional Health Americas found that approximately 18.2 million Californians — primarily in the Central Valley, L.A. and San Diego metro areas — live where conditions are suitable for local dengue transmission at least one month a year, usually July or August.
“Under moderate scenarios of climate warming and urban expansion, an additional 4.1 million residents may be at risk by mid-century,” according to the study led by UC Berkeley’s Lisa Couper.
The state’s first two locally acquired cases (meaning the people did not contract the disease while traveling) were logged in 2023. The following year, that number ballooned to 18, with 14 in Los Angeles County. The California Department of Public Health identified this local spread as the most notable recent vector-borne trend it’s tracking.
There were seven cases in 2025, and so far zero this year.
The black-and-white striped mosquitoes were first detected in 2013 in California, likely arriving via human travel and cargo transport. The same mosquito carries the diseases Zika and Chikungunya.
And there’s flea-borne typhus, which is also on the rise in Southern California. In 2025, Los Angeles County health officials confirmed 220 cases — a sharp increase over the 187 cases reported in 2024.
The reasons? More of the same. A warmer climate has expanded the flea season. And as people (and their pets) move into areas closer to wildlife, opportunities for the typhus bacteria to jump to people grows.
But there may be another reason, too, said Jim Farrar, director of the UC Statewide Integrated Pest Management Program: State laws and local ordinances that have banned certain rodenticides could be inadvertently fueling typhoid outbreaks.
Last month, a park in Fairfax in Marin County was overrun with fleas. The city had to shut down the park for several weeks. In 2001, Fairfax lawmakers enacted a ban on all herbicides, insecticides and rodenticides on public land. The state also has widespread prohibitions on several rodenticide ingredients.
“I mean, there was a good reason for this,” Farrar said. Wildlife officials had found that many of these poisons worked their way through the food chain, sickening and killing owls, raptors, coyotes and mountain lions.
But it has left an opening for some of these vector reservoir animals — such as rats, mice and others — to take advantage of their changing climate and landscape to breed and expand their ranges; which leads to more opportunity for wildlife-to-human disease transfer.
Foley, however, cautioned that trying to pinpoint any one factor of disease growth is probably shortsighted.
She said researchers trying to do that can often miss the complexity, scale, and natural variability in the systems they are investigating. Traditional approaches to studying cause-and-effect relationships are often not possible when investigating populations and ecosystems, which are large, complex and messy.
We are likely to see more of these diseases. And new ones. As people move deeper and deeper into ecosystems and territories where few, if any, had lived before, they risk interacting with pathogens that were previously confined.
“We are dysregulating ecosystems and creating more opportunities for contact,” said Foley, the UC Davis veterinarian. She pointed to news of road construction crews digging deeper and deeper into Brazil’s Amazon forest, which she said will inevitably “bring us into contact with birds and bugs” and the disease-baggage they carry.
Times staff writer Mack Baysinger contributed to this report.