The Fungus Among Us...,

scientificamerican |  We are likely to think of fungi, if we think of them at all, as minor nuisances: mold on cheese, mildew on shoes shoved to the back of the closet, mushrooms springing up in the garden after hard rains. We notice them, and then we scrape them off or dust them away, never perceiving that we are engaging with the fragile fringes of a web that knits the planet together. Fungi constitute their own biological kingdom of about six million diverse species, ranging from common companions such as baking yeast to wild exotics. They differ from the other kingdoms in complex ways. Unlike animals, they have cell walls, not membranes; unlike plants, they cannot make their own food; unlike bacteria, they hold their DNA within a nucleus and pack cells with organelles—features that make them, at the cellular level, weirdly similar to us. Fungi break rocks, nourish plants, seed clouds, cloak our skin and pack our guts, a mostly hidden and unrecorded world living alongside us and within us.

That mutual coexistence is now tipping out of balance. Fungi are surging beyond the climate zones they long lived in, adapting to environments that would once have been inimical, learning new behaviors that let them leap between species in novel ways. While executing those maneuvers, they are becoming more successful pathogens, threatening human health in ways—and numbers—they could not achieve before.

Surveillance that identifies serious fungal infections is patchy, and so any number is probably an undercount. But one widely shared estimate proposes that there are possibly 300 million people infected with fungal diseases worldwide and 1.6 million deaths every year—more than malaria, as many as tuberculosis. Just in the U.S., the CDC estimates that more than 75,000 people are hospitalized annually for a fungal infection, and another 8.9 million people seek an outpatient visit, costing about $7.2 billion a year.

For physicians and epidemiologists, this is surprising and unnerving. Long-standing medical doctrine holds that we are protected from fungi not just by layered immune defenses but because we are mammals, with core temperatures higher than fungi prefer. The cooler outer surfaces of our bodies are at risk of minor assaults—think of athlete's foot, yeast infections, ringworm—but in people with healthy immune systems, invasive infections have been rare.

That may have left us overconfident. “We have an enormous blind spot,” says Arturo Casadevall, a physician and molecular microbiologist at the Johns Hopkins Bloomberg School of Public Health. “Walk into the street and ask people what are they afraid of, and they'll tell you they're afraid of bacteria, they're afraid of viruses, but they don't fear dying of fungi.”

Ironically, it is our successes that made us vulnerable. Fungi exploit damaged immune systems, but before the mid-20th century people with impaired immunity didn't live very long. Since then, medicine has gotten very good at keeping such people alive, even though their immune systems are compromised by illness or cancer treatment or age. It has also developed an array of therapies that deliberately suppress immunity, to keep transplant recipients healthy and treat autoimmune disorders such as lupus and rheumatoid arthritis. So vast numbers of people are living now who are especially vulnerable to fungi. (It was a fungal infection, Pneumocystis carinii pneumonia, that alerted doctors to the first known cases of HIV 40 years ago this June.)

Not all of our vulnerability is the fault of medicine preserving life so successfully. Other human actions have opened more doors between the fungal world and our own. We clear land for crops and settlement and perturb what were stable balances between fungi and their hosts. We carry goods and animals across the world, and fungi hitchhike on them. We drench crops in fungicides and enhance the resistance of organisms residing nearby. We take actions that warm the climate, and fungi adapt, narrowing the gap between their preferred temperature and ours that protected us for so long.

But fungi did not rampage onto our turf from some foreign place. They were always with us, woven through our lives and our environments and even our bodies: every day, every person on the planet inhales at least 1,000 fungal spores. It is not possible to close ourselves off from the fungal kingdom. But scientists are urgently trying to understand the myriad ways in which we dismantled our defenses against the microbes, to figure out better approaches to rebuild them.