Pathogen
A microorganism or organism that causes disease by infecting a host and reproducing inside it, harming the host's health.
An organism—typically a bacterium, virus, fungus, or parasite—that invades a living host and reproduces within it, causing infection and disease. Pathogens evolved to spread between hosts and exploit their resources for survival and reproduction.
What this means in real life
When you catch a cold, the rhinovirus is the pathogen; it enters your nose and throat, replicates in your cells, and triggers sneezing and congestion as your immune system fights back.
What it isn’t
Not every microorganism is a pathogen—most bacteria on your skin and in your gut are harmless or beneficial. A pathogen specifically causes harm; a benign microbe does not.
Commonly misused online
Social media often conflates 'pathogen' with 'virus' alone, ignoring that bacteria, fungi, and parasites are equally valid pathogens. The term is also sometimes used metaphorically to mean any corrupting influence, diluting its scientific meaning.