LOS ANGELES — Epidemiologists are investigating an outbreak of valley fever that has sickened 28 workers at two large solar-power-construction sites in San Luis Obispo County, Calif.

Valley fever, also known as Coccidioidomycosis, is contracted by breathing in fungal spores released when desert soils are disturbed. The illness is prevalent in many areas of the California desert and is common among agricultural workers and construction crews. It is not contagious.

Of those who contract the illness, less than half develop flu-like symptoms that include a cough, fever, chest pains and muscle aches that can persist for weeks. In extreme cases, valley fever can cause death, usually among patients with compromised immune systems.

Officials announced they found the disease two months ago at the Topaz Solar Farm and California Valley Solar Ranch, two large-scale photovoltaic-power plants whose construction often requires considerable scraping and clearing to make way for thousands of acres of solar panels.

The Topaz project in San Luis Obispo is a 550-megawatt PV solar farm that will contain 9 million ground-mounted panels on 3,500 acres. The 250-megawatt California Valley project is on 4,700 acres. Both projects are still under construction.

The Centers for Disease Control and Prevention announced in March that valley-fever cases in the Southwest have increased by 90 percent from 1998 to 2011. California’s increase mirrors that, with reported cases growing 71 percent over the decade 2001 to 2011.