More recently, Saccharomyces has served as an agreeable model organism in the laboratory, an excellent way to explore how genes behave and cells divide and a much cheaper date than a rodent. Fungal cells turn out to be surprisingly similar to animal cells, and researchers recently determined that the fungal and animal lineages didn’t split from each other until millions of years after both had branched away from the plants.

The defining traits of a fungus are gustatory and architectural. Whereas animals ingest a meal first and then digest it internally, fungi do the reverse. After latching on to a suitable food source, they release enzymes to break down the substance into a soupy mash of sugars and amino acids, which they can then absorb through the membranes of their filamentous hyphae. Some fungi remain simple, even unicellular, but others can sprout elaborate fruiting bodies packed with billions of microscopic spores, billions of wistful homuncular fungi.

The most familiar fruiting bodies are the mushrooms, with their vivid pigments of inscrutable purpose and their still more inscrutable forms  here a swollen pink pincushion or a bird’s nest filled with eggs, there a protruding black tongue or a batch of bright butter coral. Given sufficient food and room, the filaments of a founding fungus may grow over thousands of acres of soil and persist for centuries or millennia, all the while spawning genetically identical mushrooms above ground, and biologists have argued that such hyphal masses qualify as some of the largest and most ancient organisms on Earth.

Most fungi are adapted to grow in cool or foresty temperatures, maybe 60, 70 degrees Fahrenheit, which is why the pathogens among them tend to prey on plants, or cold-blooded animals like insects, reptiles or amphibians.

Even then, most fungal diseases are not fatal, and the virulent strain that is thought to be involved in today’s mass amphibian die-offs may have been introduced into natural populations by frogs used in medical research.

With their hot body temperatures, mammals and birds suffer from few fungal diseases save those confined to the coolish epidermis. Bats are mammals, but the species now afflicted by white-nose syndrome are cave-hibernating bats, and when the bats lapse into their hibernation torpor, said David S. Blehert, a microbiologist with the United States Geological Survey’s National Wildlife Health Center in Madison, Wis., their core body temperature drops down to just a couple of degrees above cave conditions, as low as 44 degrees.

“This pathogen is treating the bats as if they were forgotten tubs of cottage cheese in the back of the refrigerator,” Dr. Blehert said. Moreover, the fungus appears to be unusually virulent. “We’re seeing in excess of 90 percent mortality at some sites,” Dr. Blehert added.

Since the disease was first identified west of Albany in March 2007, it has spread to bats in nine states and is on the cusp of reaching bat populations that aggregate in groups 300,000 strong, “the largest colonies of hibernating mammals known on the planet,” Dr. Blehert said. In an effort to block the pathogen’s passage, wildlife authorities are closing off caves to human traffic, for now the only measure they can think of to keep the wrath of Robigus at bay.