In 2008 Matthew Allender, a wildlife veterinarian affiliated with the University of Illinois at Urbana–Champaign, noticed something wrong with some of the rattlesnakes he'd been studying as part of a population-monitoring program near Carlyle, Ill. His team had found three eastern massasauga rattlesnakes (Sistrurus catenatus catenatus) with severe facial swelling and disfiguration. Allender brought them back to his lab for observation. The lesions ulcerated and progressed beneath the skin. When the snakes died just three weeks later the disease had deformed their mouths, nasal passages and fangs beyond recognition.

Allender described a fungus, Chrysosporium, as the culprit. In 2010 Allender's team found another infected snake, and since then two more. Allender doesn't yet understand how the disease is transmitted—or how to stop it.

"I've never seen anything like this in wild snakes before," Allender says. "Nor has anyone else. In almost all organisms, fungi are opportunistic pathogens that attack compromised immune systems. These were otherwise healthy snakes," however, he says. Already suffering from diminishing numbers, the snake is a candidate for U.S. Fish and Wildlife Service's threatened species list.

Unfortunately, the rattlesnakes are only the latest example of species falling prey to fungal attack in a troubling global trend noted by public health officials, zoologists and conservationists. Fungi have afflicted species as varied as amphibians, bats, arabica coffee, mangrove crabs, wheat, coral, bees, oak trees, sea turtles and even humans. (For instance, infectious meningitis is caused by a fungus.)

The most well-documented example is the lethal amphibian fungus, Batrachochytrium dendrobatidis, commonly known as chytrid. Originally reported in 1997, chytridiomycosis has infected more than 500 species of frogs and salamanders on all continents where amphibians are found, and launched half of all amphibian species into evolutionary decline. Many other species affected by fungal disease face imminent extinction, such as the European crayfish.

View a slide show of invasive fungi's trail of death and disease.

To establish whether the data scientists were gathering really did point to a dramatic shift in a deadly trend affecting numerous species, Matthew Fisher, a reader in fungal disease epidemiology at Imperial College London, and his colleagues conducted a meta-analysis of past studies available on Web of Science, an online citation index provided by Thomson Reuters, ProMED (the Program for Monitoring Emerging Diseases) and HealthMap, which monitors disease outbreaks in plant and animal hosts. Their findings revealed that fungi and funguslike pathogens (oomycetes) account for 65 percent of the pathogen-driven species loss in the past half century.

Perhaps it is unsurprising in a global economy, but human activity such as international trade and military operations have intensified the dispersal of fungal pathogens, delivering new foes to unprotected victims and introducing new evolutionary opportunities to previously harmless fungus species.

Long thought to reproduce asexually through mitosis, where each offspring is the identical clone of its parent, scientists have discovered fungi can also reproduce sexually, via meiosis. By nimbly changing their reproductive strategy in response to new environmental conditions, fungi transfer genetic advantages from both parents—just like humans do—giving their offspring a better shot at survival. They also readily hybridize (interbreed between different species), outcross (selectively breed with individuals of different strains within a species) and recombine (exchange genetic material during cell division).

Zymoseptoria pseudotritici, a pathogenic fungus that damages wheat crops, is a good example. This recent hybrid resulted directly from the coupling of two genetically distinct, nonpathogenic fungi that had been brought into contact through human trade and agricultural practices. The offspring, unlike its parents, is a killer.

"If there is some new environmental condition in which they can't thrive, some fungi change their reproductive strategy and they reproduce sexually. Fungal sex is far more common than we ever thought. In terms of sheer numbers, they're among the most successful organisms on the planet," Fisher says.

One of the most sinister weapons in fungi's survival arsenal is its ability to hide in any life-form that is being shipped from one country to another—and then to wait out poor conditions. For instance, Phytophthora ramorum, the funguslike oomycete that has caused the die-off of native oak trees in California and Oregon in the past decade, probably hitched a ride on a non-susceptible host in Asia, mostly likely a rhododendron, through the ornamental plant trade. "There were no warning bells for this disease," says Matteo Garbelotto, extension specialist at the Department of Environmental Science, Policy and Management at the University of California, Berkeley. Once the fungus arrived on the California coast, the warm weather and intermittent rainfall enabled it to move easily among hosts. "What we think happened was the pathogen moved from the rhododendron to the California bay laurel, where it can lay dormant for years. There is no direct oak-to-oak transmission," Garbelotto says.

When environmental conditions aren't perfect, phytopthora bides its time. "In dry years, they don't propagate," Garbelotto says. "Phytophthora is a zoospore, which means it goes through a swimming phase during its life cycle. If it's dry or the temperature is low, there is no outbreak. But when the conditions are right, an epidemic can go from nonexistent to affecting innumerable trees within a few weeks."

Not all fungi are bad—far from it. Without fungi we wouldn't have Penicillium which ages blue cheese, rots oranges and from which the antibiotic drug penicillin was extracted. "Without fungi, life on Earth would look very different," Fisher adds. "Forests themselves depend on fungi for their survival." Mycorrhizal, or symbiont fungi, have evolved mutualistically with plant-root vascular systems. They transfer nitrogen and phosphorus from the soil to the plant roots through these associations.

Fungi are among evolution's most successful organisms. Yet it took only a few generations for some of nature's most awe-inspiring assets to become some of its most fearsome liabilities for other species on the planet.