bacteria and protozoans can both be parasites. Feeding garlic is one of the ways you can help your fish resist parasite infections but it takes a lot of effort to get garlic to eradicate ich on fish which are already infected.
In the context of the question asked I answered parasite since it's not a bacteria.
A parasite is an organism that derives its food, nutrition and shelter by living in or on another organism. Therefore, by its very nature, it robs nutrients from its host and distracts the host's immune system from warding off disease. Within the broad term "parasite", a range of creatures can fall - worms, flukes, mites, yeast, bacteria, protozoa and viruses, just to name a few.
organisms won't harm you. Your immune system protects you against a multitude of infectious agents. However, some bacteria and viruses are formidable adversaries because they're constantly mutating to breach your immune system's defenses.
Bacteria are one-celled organisms visible only with a microscope. They're so small that if you lined up a thousand of them end to end, they could fit across the end of a pencil eraser. They're shaped like short rods, spheres or spirals. Bacteria are self-sufficient Ã¢â‚¬" they don't need a host to reproduce and they multiply by subdivision.
Viruses in it's simplest form is a capsule that contains genetic material - DNA or RNA. Viruses are even tinier than bacteria.
Protozoa are single-celled organisms that behave like tiny animals - hunter and gathering other microbes for food. Protozoa can live within your body as a parasite.