This chapter examines data of free-living microorganisms and the many single-celled animal or plant disease agents. Habitats include soil, fresh and sea water as well as animal and plant hosts. One study examined the human microbiome, including specific, mixed-species, and community TPLs. Examples of free-living species include species such as amoebae, diatoms, and foraminifera. A study comparing the distribution of marine viruses in the Pacific Ocean and the Gulf of Bothnia has identical TPLs but different levels of abundance. Disease agents include viruses, bacteria, and fungal spores. Data of their abundance are usually recorded as number per host or plant part. TPL of powdery mildew of apples illustrates the sudden decline in variance that can occur at high density when the sampler becomes saturated with all available leaf space occupied by mildew colonies