According to a 2006 study of sexually transmitted diseases, the bacterium Mycoplasma genitalium is the smallest living cell as of 2014. It measures only 200 to 300 nanometers, according to Wikipedia.
Its size categorizes Mycoplasma genitalium as the smallest of the ultramicrobacteria, a special subclass of bacteria that are notably smaller than widely accepted tiny bacteria like chlamydia and Rickettsia. Since the bacterium is to small, the study claims, culturing it for study can last from weeks to months before a sample is large enough for experimentation. Furthermore, these bacteria live inside the genital epithelial cells of its hosts, making it much more difficult to sample than pathogens that exist in the fluids of their hosts.