The smallest living things are typically considered to be certain types of microorganisms, specifically bacteria. Among them, Mycoplasma species are often cited as some of the smallest known organisms. For example, Mycoplasma genitalium has a very small genome and can be as small as 0.2 micrometers in diameter.
Another contender in the realm of the smallest living entities is the virus. However, viruses are not considered fully alive by many definitions in biology, as they require a host cell to replicate and do not carry out metabolic processes independently.
In summary, while Mycoplasma genitalium can be considered one of the smallest living organisms, the question of what constitutes the "smallest" can vary depending on the criteria used (such as size, cellular structure, or the definition of life).