pathogen

noun

1. An agent that can cause disease, especially an infectious microorganism, such as a bacterium, virus, protozoon or fungus.

   “In his first book since the 2008 essay collection Natural Acts: A Sidelong View of Science and Nature, David Quammen looks at the natural world from yet another angle: the search for the next human pandemic, what epidemiologists call “the next big one.” His quest leads him around the world to study a variety of suspect zoonoses—animal-hosted pathogens that infect humans.”