means A microorganism — such as a bacterium, virus, or fungus — that causes disease in its host.

from From Greek pathos, 'suffering' or 'disease,' joined to the suffix -gen, 'producer of' (from gignesthai, 'to be born' or 'to come into being'). So a pathogen is literally a 'disease-maker.' The word is a relatively modern scientific coinage, assembled in the late 19th century as germ theory took hold and scientists needed a name for the tiny culprits behind illness. Its cousin pathos still lives on in 'pathology,' 'sympathy,' and 'pathetic.'