Germ,microbe,bacterium,bacillus,virus:
Though not strict synonyms,are comparable because all denote organisms invisible to the naked eye,including organisms that are the causative agents of various diseases.
Germ and microbe are the ordinary nonscientific names for such an organism and especially for one that causes disease.
Bacteria,the plural of bacterium and the form commonly in general use,is often employed as the equivalent of designation of a large group of prokaryotic microbes which are found widely distributed in water,air,soil,living things,and dead organic matter,which have structural and biological characteristics distinguishing them from other unicellular microorganisms (as protozoans),and only some of which are instrumental in producing disease in man,animals,and plants.In addition to the pathogenic or disease-causing bacteria there are the saprophytic bacteria which live upon dead or decaying organic matter and which,for the most part,are beneficial in their effects which include many natural chemical processes (as fermentation,oxidation,and nitrification).
From Merriam-Webster’s Dictionary of synonyms