 Perhaps the most basic philosophical issue that awareness of microbes leads to is that of individuality: individuating where one biological entity starts and another begins. Are microbes in and on human bodies part of those bodies or not? If they are deemed a part, in what way is that parthood conceptualized? Rather than answering this question with traditional tools of mereology (see  SEP entry on mereology),  philosophers of biology have focused on biological and evolutionary criteria. In particular, the question of whether hosts and their microbes evolve as one Darwinian unit has become central to the discussion (e.g., Clarke 2013; Booth 2014; Godfrey-Smith 2015; Pradeu 2016; Roughgarden et al. 2018). Criteria for evaluating when symbiotic biological units have become a single evolutionary unit of selection are still disputed and promise considerably more philosophical discussion (see  SEP entry on individuality).  What is central to this entry is that such debates are rarely inspired by relationships between macroorganisms. The smallness and ability of microbes to invade host bodies and even cells is what marks out microorganisms as the relevant focus of philosophical attention. Their systemic effects on their environments is the other aspect that makes them such important research foci. For example, microbes in the intestines are repeatedly linked to host brain states such as emotional well-being and depression (e.g., Valles-Colomer et al. 2019).