> The case remains under investigation, but it appears that someone became infected with the dengue virus, returned home and was bitten by a mosquito that passed it on to the local resident, according to Pasadena health officials.

https://abcnews.go.com/Health/wireStory/california-officials...

It seems part of the common pattern for outbreaks of dengue viral infections in other urban centres. However California lacks a non-human primate reservoir would probably limit the risk of it becoming endemic.

There have been cases in other states too over the last 10 years. Florida, Texas and Arizona had a lot more locally acquired cases. Those might have been the first locally acquired cases in California. Maybe with climate change, as it gets warmer it becomes more prevalent.

https://www.cdc.gov/dengue/statistics-maps/historic-data.htm...

And France recently one (of someone who never travelled outside of France).