Humans cannot see, smell, or taste individual micro-organisms such as bacteria, protozoa and certain fungi and algae. Such living organisms are microscopic and too small to be seen without magnification instruments (they were first observed by Anton van Leeuwenhoek in 1675).
Some micro-organisms reproduce, replicate and swarm to form clusters large enough to be seen, smelled and tasted by humans. For example, bunches of mold spores -members of the fungi kingdom- on a loaf of bread may be seen, smelled and tasted, while an individual mold spore is undetectable.
Another example is the bacteria that cause bad breath. While not visible to the naked eye, clusters of the bacteria do have a unique taste and smell.