Much medical microbiome research focuses on health and illness as the state of the combined host-microbiome system. Concepts of health and illness, or normal and abnormal, are already problematic when just the host is taken into account. Conceptualizing a particular host-microbiome ecosystem as healthy or dysfunctional is likely to be even more fraught. This talk will discuss the issues, and suggest that microbial ecology offers several ways to inform medical thinking on this topic.