Humans are inhabited by a broad diversity of nano-sized bacteria with tiny genomes.

McLean, J.S., Bor, B., Kerns, K.A., Liu, Q., To, T.T., Solden, L., Hendrickson, E.L., Wrighton, K., Shi, W., and He, X. (2020). Acquisition and Adaptation of Ultra-small Parasitic Reduced Genome Bacteria to Mammalian Hosts. Cell Reports 32.