It's difficult to say definitively what the absolute smallest parasite is, as there are many tiny organisms that live parasitically on or within other organisms.

Here are some contenders for the title of "smallest parasite":

* Viruses: Technically, viruses are not considered living organisms, but they can be parasitic and are incredibly small. Some viruses are only a few nanometers in size, making them much smaller than bacteria.

* Nanobacteria: These are tiny, potentially self-replicating particles that are smaller than bacteria. While their existence and role as parasites are still debated, some researchers believe they may be associated with certain diseases.

* Mycoplasmas: These are bacteria that lack a cell wall and are among the smallest known bacteria. Some mycoplasma species are parasitic and can cause diseases in animals and humans.

* Rickettsia: These are bacteria that live inside other cells (such as insects) and are also quite small.

Ultimately, the definition of "smallest" can be a bit subjective. It depends on what you consider to be a parasite and whether you're looking at individual cells or entire organisms. But all of these contenders are incredibly small and illustrate the diversity and complexity of the parasitic world.