By now, you’ve probably heard that your body is teeming with bacteria. Some 100 trillion of them live on your skin, in your mouth and in the coils of your intestines. Some protect against infections and help you digest food, while others can make you seriously ill.

Fungi, viruses and protozoa call your body their home, too. Your fungal residents are less numerous than your bacteria by orders of magnitude, but as researchers are learning, these overlooked organisms play an important physiological role — and when their numbers get out of whack, they can modify your immune system and even influence the development of cancer.

A new study, published Wednesday in the journal Nature, found that fungi can make their way deep into the pancreas, which sits behind your stomach and secretes digestive enzymes into your small intestine. In mice and human patients with pancreatic cancer, the fungi proliferate 3,000-fold compared to healthy tissue — and one fungus in particular may make pancreatic tumors grow bigger.

Researchers were surprised by the presence of fungi in the typical pancreas and immense increase in their numbers in disease. “The pancreas was considered a sterile organ until a couple years ago,” said Dr. George Miller, a surgical oncologist at the New York University School of Medicine who led the study.