LONG BEACH, Calif. (AP) — Two people in Southern California have come down with dengue fever without traveling outside the United States, where the mosquito-borne illness is rare, health officials said.

A Pasadena resident was confirmed to have dengue last month but is recovering, officials said.

“This is the first confirmed case of dengue in California not associated with travel and is instead an extremely rare case of local transmission in the continental United States,” the Pasadena Public Health Department announced.

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.

On Wednesday, Long Beach officials announced another domestically contracted dengue case and said that person has recovered.

Both cities’ health departments said the risk of exposure to others was low.

Dengue is caused by several related viruses and is spread through the bite of infected Aedes mosquitoes. It is common in tropical areas and causes high fevers, headaches, nausea, vomiting, muscle pain and, in the most serious cases, internal bleeding leading to death.

About 4 billion people, or about half the world’s population, live in areas where dengue is a risk, and each year there are up to 400 million infections and about 40,000 deaths, according to the U.S. Centers for Disease Control and Prevention.

The infection rate worldwide has been rising, prompting new efforts to fight it.

However, dengue is rare in the U.S. and its territories, with only 583 locally acquired cases reported so far this year, according to CDC data: 520 in Puerto Rico, 62 in Florida and one in Texas.

The new California cases were not part of that count.