any disease-producing agent (especially a virus or bacterium or other microorganism)
"The new test can quickly identify which pathogen is causing your persistent fever so doctors can prescribe the right treatment."
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.
"The doctor explained that the patient's fever was caused by a specific pathogen that only became harmful once it multiplied within the body to dangerous levels."