The accidental pathogen
Deep inside a self-heating compost heap, temperatures can climb past 60°C (140°F), creating an extreme environment where few organisms can survive. This is the natural ecological niche for Aspergillus fumigatus, a fungus that helps nature by recycling carbon and nitrogen from decaying organic matter. It didn't evolve to infect animals. Instead, it evolved to withstand the intense heat of decomposition. This ability is called thermotolerance. The fungus can grow at temperatures up to 55°C and its spores can survive temperatures as high as 70°C (158°F).
This adaptation had an unintended consequence for humans. Our core body temperature, approximately 37°C, falls comfortably within the fungus's optimal growth range. Most fungal species cannot tolerate such heat, which provides mammals with a "thermal restriction zone," a natural defense against fungal infection. A. fumigatus is an exception. Its evolution in hot compost pre-adapted it for the warmth of a mammalian body, making it an accidental but effective opportunistic pathogen.
The fungus propagates by releasing thousands of microscopic spores, called conidia, from each colony. These spores measure just 2 to 3 micrometers in diameter, small enough to be easily carried on air currents and bypass the body's upper respiratory defenses to lodge deep within the lungs. Every person on Earth inhales hundreds of A. fumigatus conidia daily. For individuals with healthy immune systems, these spores are quickly identified and eliminated.
From compost to clinic
In people with compromised immune systems, the story is different. This includes patients with leukemia, those who have received organ or stem cell transplants, or individuals with AIDS. In these vulnerable hosts, the inhaled conidia can germinate into their active, filamentous form, called hyphae, and cause a life-threatening infection known as invasive aspergillosis. The infection begins in the lungs and can spread through the bloodstream to other organs, including the brain.
Invasive aspergillosis is difficult to diagnose and has a high mortality rate. Globally, it causes over 250,000 infections annually. The crude annual mortality rate is estimated to be around 85%. For bone marrow transplant recipients who contract the disease, the case-fatality rate can be as high as 86.7%.
Scientists are increasingly concerned about the connection between climate change and fungal pathogens. As global average temperatures rise, the thermal gap between the environment and our body temperature narrows. This warming trend could allow more heat-tolerant fungi to emerge as new human pathogens. The multidrug-resistant yeast Candida auris is suspected to be the first example of a new pathogen emerging due to climate change. At the same time, extreme weather events like storms and wildfires can aerosolize fungal spores, increasing the risk of exposure.