https://phys.org/news/2019-01-oak-trees-dying-scientists-infectious.html

A pathobiome is essentially a community of interacting bacteria that together cause a disease in animals and plants. Acute Oak Decline, for example, arises from several bacteria which together degrade the trees' vascular tissues, preventing transport of water and nutrients to the branches and leaves. We have recently identified two bacteria as causative agents of Acute Oak Decline – Brenneria goodwinii and Gibbsiella quercinecans – while others, including Lonsdalea britannica and Rahnella species, have been detected, although their role is currently unclear.

Alone, the bacteria which form the pathobiome are harmless, or are less severe. But when alongside other organisms the likelihood, incidence and severity of disease is increased. This gives rise to "emergent property", meaning that the disease causing property is greater than the sum of the combined organisms.