“Bug” is sometimes used to refer to an illness caused by a bacterium or virus, as in “I can’t come into work today; I must have caught that [flu/stomach] bug th

“Bug” is sometimes used to refer to an illness caused by a bacterium or virus, as in “I can’t come into work today; I must have caught that [flu/stomach] bug that’s going around”. It’s not such a stretch to use it colloquially to refer to the microorganism itself.