Coccidioidomycosis is a fungal infection caused by inhaling spores of Coccidioides from desert soil, most commonly in the American Southwest. Often called Valley fever, it causes roughly 20,000 reported cases per year in the United States, though the true number is thought to be several times higher because many cases go undiagnosed or unreported. Most people who breathe in the spores either fight off the infection without symptoms or develop a flu-like illness that resolves on its own, but a fraction of cases progress to severe, chronic, or disseminated disease that can be debilitating or fatal.
The Fungus and How It Infects
Two closely related species cause the disease: Coccidioides immitis and C. posadasii. They look identical under a microscope, and their proteins are over 90 percent alike. The main practical difference between them is where they live. C. immitis predominates in central and southern California, while C. posadasii occupies desert regions of Arizona, Nevada, New Mexico, west Texas, Mexico, and parts of Central and South America.1PubMed Central. Coccidioides immitis and posadasii; A review of their biology, genomics, pathogenesis, and host immunity From a patient’s perspective, the species distinction rarely matters because the diseases they cause are clinically indistinguishable and treated the same way.
In soil, Coccidioides grows as a mold, forming chains of barrel-shaped spores called arthroconidia. When the soil is disturbed, these microscopic spores become airborne. A single breath can deliver them deep into the lungs. Once inside a warm-blooded host, the fungus undergoes a dramatic shape-shift: each spore swells into a large, round structure called a spherule, which matures over roughly five to ten days and fills with hundreds of smaller cells called endospores. When a spherule ruptures, those endospores scatter into surrounding tissue, each capable of growing into a new spherule, continuing the cycle of infection.2Medical Mycology. The endozoan, small-mammal reservoir hypothesis and the life cycle of Coccidioides species
Where It Lives in the Environment
For decades, researchers struggled to find Coccidioides in the wild because the fungus is patchy and hard to culture from soil. A growing body of work has focused on animal burrows as hotspots. In one Arizona study, the odds of detecting Coccidioides in soil from animal burrows were about three times higher than in soil outside burrows.3PubMed Central. Investigating the Role of Animal Burrows on the Ecology and Distribution of Coccidioides spp. in Arizona Soils A longer-term experiment using fenced plots that excluded rodents found an even starker difference: the odds of detecting the fungus in burrow soils were about 19 times higher than in surface soils, and roughly two and a half times higher where rodents were present compared to where they were kept out. The study estimated that about three-quarters of the association between rodents and fungal detection was explained by the rodents physically creating burrows.4bioRxiv. Small mammals and their burrows shape the distribution of Coccidioides in soils: a long-term ecological experiment
The implication is that the fungus thrives in the microenvironment rodent burrows provide: stable temperature, organic matter from droppings and carcasses, and protection from the sun’s ultraviolet light. When burrowing animals kick up soil, or when construction equipment plows into burrow-riddled ground, spores can become airborne in large numbers. This ecology is part of why Valley fever cases cluster around land disturbance, not just arid climate.
Symptoms and How Long They Last
An estimated 60 percent of people who inhale Coccidioides spores never develop symptoms at all. Among those who do get sick, symptoms typically appear one to three weeks after exposure and resemble a bad respiratory infection: cough, fever, chest pain, body aches, and fatigue. Doctors in endemic areas sometimes refer to the classic presentation as “desert rheumatism” because it can include fever, painful red bumps on the shins (erythema nodosum), and joint pain alongside the respiratory complaints.5PubMed Central. Valley Fever: Pathogenesis and Evolving Treatment Options
What often catches patients off guard is how long recovery takes, even in straightforward cases. A study that tracked people with confirmed lung-limited Valley fever found that fatigue in the early weeks was worse than levels reported by patients with multiple sclerosis. Physical functioning gradually improved, with most participants reaching levels comparable to the general population by about week 12. But the fatigue lingered, and patients who needed antifungal treatment took even longer to bounce back. Mental and emotional health scores were not as badly affected as physical ones, though the overall drag on quality of life was substantial and persisted for months.6Medical Mycology. Uphill both ways: Fatigue and quality of life in valley fever
When the Infection Spreads Beyond the Lungs
In a small percentage of cases, the fungus escapes the lungs and spreads to other parts of the body. This is called disseminated coccidioidomycosis, and it can affect the skin, bones, joints, and the membranes surrounding the brain and spinal cord (coccidioidal meningitis). Skeletal involvement accounts for roughly half of disseminated infections, with the spine being a particularly common site. Bone destruction and spread into surrounding soft tissue can cause severe pain and limit mobility.7PubMed Central. Coccidioidomycosis Osteoarticular Dissemination Lung cavities that persist after the initial infection can also develop, sometimes causing bleeding or becoming secondarily infected by bacteria or other fungi.
Meningitis is the most feared complication. Without treatment it is almost always fatal, and even with lifelong antifungal therapy, outcomes can be poor. Patients with coccidioidal meningitis typically require treatment indefinitely, because stopping medication leads to relapse in a high proportion of cases.
Who Faces the Highest Risk
Anyone who breathes in the spores can get Valley fever, but certain groups face a much steeper path. People who work outdoors in endemic areas are an obvious category. Agricultural workers, construction crews, archaeologists, prison employees, and wildland firefighters have all been involved in documented outbreaks.8PubMed Central. Valley Fever Is an Occupational Illness: Let’s Get the Data to Prevent It One investigation into a coccidioidomycosis outbreak among inmate wildland firefighters in California highlighted the danger of hand-tool work in dusty conditions where the fungus proliferates, and recommended that employers provide training, limit dust exposure, and implement respiratory protection programs.9PubMed Central. Coccidioidomycosis outbreak among inmate wildland firefighters: California, 2017
Pregnancy significantly raises the risk of dissemination. Pregnant women experience disseminated disease at a rate estimated to be 40 to 100 times higher than men, with the risk greatest during the later stages of pregnancy.10PubMed Central. Coccidioidomycosis in pregnancy: Case report and literature review of associated placental lesions11Clinical Infectious Diseases. Coccidioidomycosis During Pregnancy: A Review and Recommendations for Management The mechanism is thought to involve the immune suppression that naturally occurs during pregnancy, possibly combined with hormonal changes that may directly enhance fungal growth.
People with weakened immune systems from organ transplants, HIV/AIDS, or immunosuppressive medications are also at elevated risk for severe or disseminated disease. Diabetes is another recognized risk factor, though the increase in risk is less dramatic than in the groups above.
Racial and Ethnic Disparities
Studies going back decades have reported that Black and Filipino individuals face higher rates of disseminated coccidioidomycosis compared to white individuals. Black patients have been reported to have odds of disseminated disease as high as five to ten times those seen among white patients, and this increased risk persisted even after controlling for income.12PLoS Pathogens. Race and ethnicity: Risk factors for fungal infections? Filipino individuals also show elevated rates of hospitalization and severe disease. These disparities were first recognized during World War II, when military recruits of different backgrounds trained at desert bases in California and fell ill at strikingly different rates.
The question of why these disparities exist is complicated. Part of the explanation appears to be genetic: specific immune-system gene variants have been linked to susceptibility. One early study identified a particular HLA class II allele that predisposed people across multiple ethnic groups to severe disseminated disease, and found that different protective alleles operated in different populations.13Emerging Infectious Diseases. Influence of Host Genetics on the Severity of Coccidioidomycosis More recent work has identified variants in genes governing innate immune pathways that may account for a meaningful portion of the disease spectrum, including who progresses to dissemination and who does not.14PubMed Central. Genetic and Other Determinants for the Severity of Coccidioidomycosis: A Clinician’s Perspective But social factors like occupational exposure, access to healthcare, and diagnostic delay also play roles that remain difficult to fully disentangle from biology. A review of the literature on coccidioidomycosis in African Americans noted that most of the published studies linking race to severity have methodological limitations.15PubMed Central. Coccidioidomycosis in African Americans
Diagnosis Is Often Delayed
Valley fever is frequently misdiagnosed as bacterial pneumonia, the flu, or a generic respiratory virus, especially in patients who have recently moved to or traveled through an endemic area and whose doctors may not be thinking about fungal infections. In one community-acquired pneumonia study, a large majority of Valley fever patients were initially prescribed antibiotics before the correct diagnosis was made. Although serologic testing can confirm the diagnosis, results are often delayed by three to five days, during which unnecessary antibiotic courses may be started or completed.16PubMed Central. Coccidioidomycosis as a Common Cause of Community-acquired Pneumonia
The problem worsens outside endemic regions. A patient who picked up spores during a vacation in Arizona and develops a cough two weeks later in Minnesota may cycle through several rounds of antibiotics and imaging before anyone considers a fungal cause. That delay matters, because while most cases resolve without antifungal drugs, catching the minority that need treatment earlier can reduce complications. Blood antibody tests and direct microscopy of tissue samples are the mainstays of diagnosis. Imaging can show lung nodules or cavities but cannot distinguish Valley fever from other infections or even from cancer without additional testing.
Treatment
Most primary lung infections do not require antifungal medication and resolve with time and symptom management.17PubMed Central. Valley Fever: Pathogenesis and Evolving Treatment Options The decision to treat is based on the severity and duration of symptoms, whether the patient belongs to a high-risk group, and whether there are signs that the infection is progressing or spreading.
When treatment is needed, oral antifungal medications in the triazole class, primarily fluconazole and itraconazole at 400 milligrams daily, are the standard. These drugs have transformed the management of Valley fever over the past few decades but have significant limitations: treatment courses for disseminated disease often last months to years, and relapse after stopping therapy is common. For patients with severe disease, including respiratory failure or rapidly progressing infection, intravenous amphotericin B remains the go-to, though its side effects are harsh.18PubMed Central. THE TREATMENT OF COCCIDIOIDOMYCOSIS Coccidioidal meningitis, as noted above, typically requires lifelong fluconazole to prevent relapse.19Clinical Infectious Diseases. Practice Guidelines for the Treatment of Coccidioidomycosis
Newer antifungal agents, including olorofim and a Valley fever-specific vaccine candidate, are in various stages of development, but as of mid-2025 there is no approved vaccine. The fungal biology that makes Coccidioides so effective at evading the immune system (the thick-walled spherule is remarkably resistant to destruction by immune cells) has made vaccine development an ongoing challenge.
Climate Change and the Expanding Endemic Zone
Valley fever case counts have climbed in recent decades, and climate change is one of the suspected drivers. The relationship is not as simple as “hotter means more cases.” Drought, paradoxically, can suppress cases during the dry period itself but fuel a rebound afterward. A modeling study in California estimated that drought between 2012 and 2016 initially averted over 2,300 cases, but the two years following the drought produced roughly 2,650 excess cases, more than offsetting the decline. A similar rebound pattern followed the 2007–2009 drought.20The Lancet Planetary Health. Influence of drought on coccidioidomycosis in California: a modelling study The leading explanation is that drought kills off competing soil microbes, giving Coccidioides less competition. When the rains return, the fungus grows prolifically, and as the surface dries out again, spores blow into the air.
Beyond this wet-dry cycle, the entire endemic zone is predicted to shift northward as temperatures rise and arid conditions spread. States that currently see few or no cases may enter the endemic zone by mid-century.21PubMed. Valley fever under a changing climate in the United States This has implications for physicians and public health systems in regions that have never had to think about fungal pneumonia.
The Dust Storm Debate
If you have ever heard of Valley fever, you have probably heard it linked to dust storms. The picture is muddier than the popular narrative suggests. A large-scale analysis of monthly and weekly case data found no consistent statistical link between dust storms and coccidioidomycosis case patterns, comparing periods following dust storms to non-dust-storm conditions.22PubMed Central. No Consistent Link Between Dust Storms and Valley Fever (Coccidioidomycosis) That finding created a stir, because the intuitive connection seems obvious: wind picks up contaminated dirt, people breathe it in, and they get sick.
The response from other researchers has been that not all dust events are equal. A haboob sweeping across Phoenix may carry soil from areas where Coccidioides is absent, while a localized windstorm near a contaminated construction site may expose a small number of people to a high concentration of spores.23PubMed Central. Dust Storms, Valley fever, and Public Awareness Research in El Paso, Texas, found that Valley fever incidence was positively correlated with particulate matter levels from the prior month, and that extreme weather conditions involving high temperatures, strong wind gusts, and elevated dust readings were associated with higher case counts, though these extreme conditions only occurred roughly once every one to three years.24PubMed. The ascending trend of valley fever in El Paso, Texas and its association with regional meteorological and dust factors The emerging consensus is that dust is a viable vehicle for the fungus, but broad-scale dust storms are too blunt a measure to predict individual-level risk. The soil the dust originated from matters more than the storm itself.
Economic Costs
Valley fever is expensive. An incidence-based cost analysis estimated that the roughly 10,400 cases diagnosed in Arizona in 2019 carried total lifetime costs of about $736 million. Direct medical costs accounted for over 90 percent of that figure. A case of uncomplicated pneumonia cost around $23,000 in direct expenses and $1,300 in lost wages. Disseminated disease, by contrast, ran to about $1.26 million in direct costs and $137,000 in lost income per patient.25PubMed Central. Clinical and Economic Burden of Valley Fever in Arizona: An Incidence-Based Cost-of-Illness Analysis
Nationally, one projection estimated annual total costs (including medical expenses, lost income, and broader economic welfare losses) at about $3.9 billion per year during the 2000–2015 baseline period. Under a high-emissions climate scenario with population growth factored in, those costs could rise by roughly 164 percent by 2050 and up to 380 percent by 2090, potentially reaching $18.5 billion per year by the end of the century.26PubMed Central. Economic Valuation of Coccidioidomycosis (Valley Fever) Projections in the United States in Response to Climate Change These projections depend heavily on how much the endemic zone expands and how effectively public health systems adapt, but the direction of the trend is clear.
Valley Fever in Dogs
Humans are not the only ones affected. Dogs are highly susceptible to Coccidioides and may be even more commonly infected than people in endemic areas, likely because they spend more time with their noses in dirt. Canine coccidioidomycosis typically presents as a persistent cough, lethargy, and lameness when bones or joints are involved. Treatment mirrors the human approach (long courses of oral antifungal medication), and some dogs require lifelong therapy.
The financial burden on pet owners is substantial. One estimate placed the annual cost of diagnosing and treating uncomplicated pulmonary Valley fever in dogs across six southwestern states at roughly $74 to $92 million (in 2022 dollars), amounting to roughly 9 to 11 percent of a typical owner’s discretionary income during the first year of diagnosis and treatment.27PubMed Central. The costs of the diagnosis and treatment of canine coccidioidomycosis in endemic regions, USA, 2022 For many families in Arizona, Valley fever in a pet is one of the hidden costs of living in the desert.
A Brief History of Discovery
The disease was first identified in 1892 by a medical student in Argentina, and for its first few decades it was assumed to be rare and almost universally fatal, because the only recognized cases were the disseminated ones. It took decades of work, largely by clinicians and scientists in California’s San Joaquin Valley, to realize that the severe cases were the tip of a much larger iceberg and that mild or asymptomatic infection was common. Key discoveries came during two periods of mass population movement into endemic areas: the 1930s Dust Bowl migration and the 1940s wartime establishment of military bases, prisoner-of-war camps, and internment camps in the desert Southwest.28PubMed. The early history of coccidioidomycosis: 1892-1945 Those events brought large numbers of immunologically naive people into contact with the fungus for the first time, producing clusters of illness that allowed researchers to map the disease’s full spectrum.
That same pattern, newcomers to endemic soil getting sick at disproportionate rates, continues today. People who relocate to Phoenix, Tucson, or the Central Valley of California from non-endemic regions lack even the partial immunity that long-term residents may have built up through subclinical exposures. For anyone moving to the desert Southwest, being aware that Valley fever exists and mentioning recent travel history to a doctor during respiratory illness is one of the simplest and most effective things you can do to avoid a delayed diagnosis.