bug: specifically, a kind of insect with piercing and sucking mouthparts (family Hemiptera), but ubiquitously used as an inaccurate term for ‘invertebrate’, ‘illness’, ‘microorganism’, ‘computer fault’ and much more. Unless it is being used to describe a hemipteran insect, it should be avoided unless the writing is colloquial and the context makes the meaning clear
germ: another imprecise term used in writing about science, usually to refer to a pathogen (a microorganism that can cause disease). Germ cells are cells that give rise to the gametes of a sexually reproducing organism
microbe and microorganism: synonymous terms that refer to any microscopic organism, either single celled or multicellular, including bacteria, fungi, protozoa and algae. Viruses are sometimes considered to be microorganisms (although it can be argued that viruses are not living organisms). Microbe or microorganism, rather than bug or germ, is the preferred collective term when discussing microscopic agents in the context of health and disease