Valley fever is a lung infection caused by inhaling spores of Coccidioides, a fungus that lives in the dry soils of the American Southwest, parts of Mexico, and Central and South America. Its medical name is coccidioidomycosis, and most people who catch it never realize they have it because the infection clears on its own. But for a meaningful minority, valley fever causes weeks or months of debilitating illness, and in rare cases it spreads beyond the lungs into bones, joints, or the brain, becoming genuinely dangerous. The disease is increasing, the region where it occurs is expanding, and it is frequently misdiagnosed as ordinary pneumonia, which makes it worth understanding even if you don’t live in Arizona or California today.
How You Catch It
Coccidioides is a soil-dwelling fungus that exists as a mold in dirt. When soil is disturbed, whether by wind, construction, farming, or even just a car driving down a dusty road, tiny asexual spores called arthroconidia become airborne and can be inhaled. Once in the lungs, those spores undergo a dramatic transformation. They morph into large, multicellular structures called spherules, which eventually burst and release hundreds of smaller endospores, each capable of growing into another spherule. That cycle of growth and rupture is what drives the infection forward inside the body.1Europe PMC. Dust devil: the life and times of the fungus that causes valley Fever
You cannot catch valley fever from another person, from a pet, or from eating contaminated food. The only route in is breathing in spores from the environment. That makes it fundamentally different from the infections people typically worry about. There is no handwashing protocol that prevents it, no social distancing that helps. If you are in an area where Coccidioides lives in the soil and dust gets kicked up, you are exposed.
Wind alone can aerosolize the spores, but the fungus’s large spore size means strong gusts are usually needed if no physical soil disturbance is involved. Research suggests that without direct digging or scraping, wind speeds above roughly 45 miles per hour would be required to launch the spores from undisturbed ground. However, a process called saltation, where wind-driven sand grains bounce along the surface and knock smaller particles into the air, can get spores airborne at much lower wind speeds.2Oxford Academic (Medical Mycology). Hypothesis on the dust-borne atmospheric transport of Coccidioides, causative fungal pathogen for coccidioidomycosis (Valley fever) That means even moderate dust storms or windy days in endemic areas carry some risk.
What It Feels Like
Most people infected with Coccidioides develop no symptoms at all or experience something indistinguishable from a mild cold. Of those who do get sick, the illness typically looks like community-acquired pneumonia: fever, cough, chest pain, fatigue, and sometimes a rash. Symptoms usually appear one to three weeks after exposure. Some people develop joint aches severe enough that valley fever earned the nickname “desert rheumatism.” A distinctive skin reaction called erythema nodosum, which produces painful red bumps on the shins, sometimes appears and is actually considered a sign that the immune system is fighting the infection effectively.
For most symptomatic patients, these problems clear up within weeks to a couple of months without any specific treatment. The trouble is that the illness can drag on. Fatigue in particular may linger for months, and some patients develop persistent lung nodules, which are small spots visible on chest imaging. These nodules can mimic lung cancer on a scan, creating real diagnostic headaches. Research comparing valley fever nodules to cancer nodules found that the two can sometimes be distinguished by their shape and features: valley fever nodules tend to have smoother borders, are more likely to contain cavities, and often have smaller satellite nodules nearby, while cancer nodules are more commonly spiculated and appear alongside signs of chronic lung disease.3PubMed Central. Differentiating Lung Nodules Due to Coccidioides from Those Due to Lung Cancer Based on Radiographic Appearance Still, the overlap is enough that biopsies and additional testing are often required.
When It Spreads Beyond the Lungs
In a small percentage of cases, the infection escapes the lungs and disseminates through the bloodstream to other parts of the body. The most common destination is the skeleton. Bone and joint involvement accounts for roughly half of all disseminated infections, with the spine being the site the fungus targets most often. Left untreated, skeletal valley fever tends to progress to bone destruction and can spread into adjacent joints, tendons, and soft tissues, causing severe pain and limiting mobility.4PubMed Central. Coccidioidomycosis Osteoarticular Dissemination
The most feared complication is coccidioidal meningitis, an infection of the membranes surrounding the brain and spinal cord. This form is life-threatening and typically requires lifelong antifungal treatment. Disseminated disease of any kind demands aggressive medical intervention and close follow-up, often for years.
Why It Is So Often Misdiagnosed
Valley fever’s biggest clinical problem may be that doctors outside endemic areas, and even some inside them, don’t think of it. The symptoms overlap almost completely with bacterial pneumonia, influenza, and COVID-19. A patient shows up with cough, fever, and fatigue; the doctor prescribes antibiotics or sends them home to rest. When the antibiotics don’t work and the symptoms persist, additional rounds of treatment may follow before anyone orders a valley fever test.
A study of patients in Tucson, Arizona, one of the most heavily affected cities in the country, found that among 276 people eventually diagnosed with valley fever, 246 experienced a delay in diagnosis, with a median delay of 23 days. That diagnostic lag was associated with nearly $590,000 in avoidable healthcare costs across the group, much of it driven by unnecessary courses of antibacterial drugs.5PubMed Central. Delays in Coccidioidomycosis Diagnosis and Associated Healthcare Utilization, Tucson, Arizona, USA If that kind of delay happens in Tucson, where clinicians encounter valley fever regularly, the situation is likely worse in places where the disease is less familiar.
Even when doctors do order the right tests, the standard methods have real limitations. The most widely used confirmatory approach detects antibodies against the fungus. But the false-negative rate for these antibody tests can reach 50 to 70 percent in early infection, and roughly 5 percent of symptomatic patients never produce detectable antibodies at all.6PubMed Central. Application of immunosignatures for diagnosis of valley fever That means a negative test does not rule it out, and patients who continue to feel sick after a negative result often need repeat testing or alternative diagnostic approaches.
Who Faces the Greatest Risk
Anyone who breathes in Coccidioides spores can develop valley fever, but several factors increase the odds of severe or disseminated disease. Older age, diabetes, and a suppressed immune system, whether from HIV, organ transplant medications, or other conditions, are well-established risk factors for complications.7PLOS Pathogens. Race and ethnicity: Risk factors for fungal infections?
Certain racial and ethnic groups face disproportionately severe outcomes. Research has repeatedly found that Black or African American individuals and Filipino individuals are at elevated risk for disseminated disease and hospitalization. For Black Americans, the odds of disseminated valley fever have been estimated at five to ten times higher than for white individuals, a disparity that persists even after controlling for income.8PLOS Pathogens. Race and ethnicity: Risk factors for fungal infections? One observed biological difference is that Black patients with valley fever are less likely to develop erythema nodosum, the shin rash mentioned earlier, which is associated with a protective immune response. Whether the underlying disparity is primarily immunological, socioeconomic, or some combination remains an active area of study.9PubMed Central. Coccidioidomycosis in African Americans
Pregnancy also increases risk. Hormonal and immune changes during pregnancy can make the infection more likely to disseminate, and treatment options become more complicated because some antifungal drugs pose risks to the developing fetus.
Occupational Exposure
Because the fungus enters the body through disturbed soil, people who work outdoors in endemic areas face the highest exposure. Construction workers, agricultural laborers, archaeologists, wildland firefighters, and military personnel stationed at desert bases have all been involved in documented outbreaks.10PubMed Central. Valley Fever Is an Occupational Illness: Let’s Get the Data to Prevent It Prison employees and inmates at facilities located in endemic regions have also been disproportionately affected.
A risk assessment looking at agricultural workers found that daily exposure risk varied enormously depending on the task and whether protective equipment was used. Workers planting by hand while wearing an N95 mask had far lower risk than workers operating harvesting machinery without a mask.11PubMed Central. A microbial risk assessor’s guide to Valley Fever (Coccidioides spp.): Case study and review of risk factors That difference highlights both the role of dust generation in transmission and the meaningful protection that proper respirators can provide. Despite this, valley fever is not consistently tracked as an occupational illness in many states, which limits the data available for prevention efforts.
Treatment
Most valley fever cases resolve without antifungal medication. For mild infections, doctors take a watchful-waiting approach, treating symptoms like pain and fever while monitoring for signs that the disease is worsening. If the infection doesn’t clear on its own or if the patient is at high risk for complications, antifungal drugs become the standard of care. Fluconazole is the most commonly used first-line treatment. For severe or life-threatening infections, particularly disseminated disease or meningitis, amphotericin B, a more potent but also more toxic antifungal, may be necessary.12PubMed Central. Valley Fever: Pathogenesis and Evolving Treatment Options
Treatment duration varies widely. A straightforward pulmonary case might need three to six months of fluconazole. Disseminated bone or joint disease can require a year or more of therapy, sometimes indefinitely. Coccidioidal meningitis is treated for life. There is no vaccine currently available, though research into one has been ongoing for decades. Progress has been slow partly because the immune response to Coccidioides is complex and partly because the population affected, while large in absolute numbers, is geographically concentrated enough that vaccine development hasn’t attracted the commercial investment seen for more widespread infections.13PubMed Central. Immune Response to Coccidioidomycosis and the Development of a Vaccine
Where Valley Fever Happens and Where It Is Heading
Two species of Coccidioides cause valley fever. C. immitis is concentrated in California’s Central Valley and the desert regions of Southern California and Baja California. C. posadasii is found more broadly across Arizona, Nevada, New Mexico, West Texas, Mexico, and Central and South America. The two species are genetically distinct but cause clinically identical disease and cannot be told apart by standard lab tests.14PubMed Central. Coccidioides immitis and posadasii; A review of their biology, genomics, pathogenesis, and host immunity
The geography of valley fever is changing. Climate modeling suggests that as temperatures rise and precipitation patterns shift across the western United States, the zone where Coccidioides can thrive will expand considerably. Under a high-warming scenario, one study projects the endemic area could more than double by 2100, with the number of affected states rising from 12 to 17 as the fungus pushes into northern states like Idaho, Wyoming, Montana, and the Dakotas. Annual valley fever cases in that scenario could increase by roughly 50 percent.15PubMed Central. Expansion of Coccidioidomycosis Endemic Regions in the United States in Response to Climate Change More recent analyses confirm this northward trend is already underway, with incidence climbing in Nevada, Utah, New Mexico, and Texas alongside the traditional hotspots of Arizona and California.16PubMed. Valley fever under a changing climate in the United States
Weather patterns within existing endemic areas also matter. Research in California supports a “grow and blow” hypothesis: wetter-than-normal winters allow the fungus to proliferate in moist soil, and when unusually dry summers follow, more spores get carried into the air. The sharp rebound in cases when rains return after a drought more than offsets the decreases seen during the drought itself, meaning each cycle of drought and recovery amplifies overall risk.17PubMed Central. Valley Fever: Fine Mineral Dust Modeling Points to High-Risk Regions and Seasons in California As droughts become more frequent under climate change, these boom-and-bust cycles in valley fever cases are expected to intensify.
The Economic Toll
Valley fever is expensive. An analysis focused solely on Arizona estimated total lifetime costs of $736 million for the roughly 10,400 cases diagnosed in the state in 2019. Direct medical costs, including hospitalizations, outpatient visits, and medications, accounted for over 90 percent of those expenditures. For a straightforward pulmonary case, lifetime direct costs averaged about $23,200. For disseminated disease, costs jumped to over $1.26 million per person.18PubMed Central. Clinical and Economic Burden of Valley Fever in Arizona: An Incidence-Based Cost-of-Illness Analysis
Looking nationally and projecting forward, the picture gets grimmer. One economic analysis estimated total annual costs of valley fever in the United States at about $3.9 billion per year during the 2000 to 2015 baseline period, factoring in medical costs, lost income, and broader economic welfare losses. Under high-emission climate scenarios, those annual costs could rise to roughly $18.5 billion by the end of the century.19PubMed Central. Economic Valuation of Coccidioidomycosis (Valley Fever) Projections in the United States in Response to Climate Change These projections account for both the expected increase in cases and the effects of population growth in the expanding endemic zone.
Valley Fever in Dogs and Other Animals
Humans are not the only ones affected. Dogs are highly susceptible to valley fever, especially those that spend time outdoors in endemic regions. Dogs dig, sniff the ground, and breathe at exactly the height where dust is thickest, making exposure almost unavoidable in affected areas. Symptoms in dogs mirror those in people: coughing, lethargy, lameness from bone involvement, and in severe cases, seizures when the infection reaches the brain.20PubMed. The costs of the diagnosis and treatment of canine coccidioidomycosis in endemic regions, USA, 2022
Treatment for dogs also involves long courses of antifungal medication, often fluconazole, and some dogs require treatment for life. The financial burden on pet owners can be significant. Veterinarians in Arizona and Southern California see valley fever in dogs frequently enough that it is part of the routine differential diagnosis for any dog presenting with a chronic cough or unexplained lameness. Cats, livestock, and various wildlife species can also be infected, though dogs bear the largest burden among domestic animals.
Protecting Yourself in Endemic Areas
There is no vaccine and no way to sterilize the soil, so prevention comes down to reducing how much contaminated dust you inhale. If you live in or visit the Southwest, a few practical steps can help:
- Limit dust exposure: Stay indoors and close windows during dust storms. Avoid outdoor activities on particularly windy days, especially in undeveloped desert areas.
- Wear respiratory protection: If your job involves disturbing soil (construction, agriculture, landscaping), an N95 or better respirator meaningfully reduces inhaled spore counts.
- Wet down soil: Before digging or grading, wetting the ground can suppress dust. This is standard practice on some construction sites in Arizona.
- Pay attention to seasonal risk: Late summer and fall, when soils are driest and winds pick up, tend to be peak exposure seasons in most endemic areas.
None of these measures eliminates risk entirely. People who have lived in the desert for decades sometimes develop valley fever for the first time. But understanding where the fungus lives and how it gets airborne gives you a realistic basis for deciding when extra caution is warranted, particularly if you fall into one of the higher-risk groups for severe disease.
How the Disease Got Its Name
The name “valley fever” comes from California’s San Joaquin Valley, where outbreaks were first recognized on a large scale in the 1930s. The disease itself was first described in 1892 by a medical student in Argentina, who mistakenly thought the organisms he saw in tissue samples were parasites rather than fungi. It took decades of work, much of it at Stanford and by clinicians in California’s Central Valley, to piece together that this was a soil fungus, that it caused both mild and life-threatening disease, and that the mild respiratory illness people called “valley fever” was the same pathogen responsible for the disseminated disease that was killing patients.21PubMed. The early history of coccidioidomycosis: 1892-1945 Migration to California’s farming regions during the Dust Bowl era and the establishment of military bases in the desert during World War II brought large numbers of immunologically naive people into contact with the fungus for the first time, producing waves of illness that finally forced the medical establishment to take the disease seriously.