Any disease-producing agent (especially a virus or bacterium or other microorganism) WordNet 3.0 & Open English WordNet

In biology, a pathogen, in the oldest and broadest sense, is any organism or agent that can produce disease. Wikipedia

Microorganism in the widest sense that causes disease in its host. Wikipedia Disambiguation

Biological entity that causes disease in its host, which is typically an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus Wikidata

Agents producing or capable of producing disease. OmegaWiki

Any disease-producing agent or microorganism. OmegaWiki

Any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa or fungi. Microorganisms are not considered to be pathogenic until they have reached a population size that is large enough to cause disease. Wiktionary

An organism or substance that causes disease. Wiktionary (translation)