The Lancet has started a series about "syndemics" (may require a subscription). Not a term I have come across before. But not a new one, either.
It is to do with the interaction of diseases, and environmental and other factors. The diseases or conditions have an influence on each other, and this is what differentiates syndemics from multimorbidity or comorbidity.
The term was devised by the anthropologist Merrill Singer, whose book Introduction to syndemics appeared in 2009. Singer developed the "emergent concept" of syndemics "as part of an ongoing effort to rethink the public health and social scientific understanding of disease so that it focused attention on the multifaceted interactions that occur among the health of a community, political and economic structures, and the encompassing physical and social environment." (Singer, 2009, p. xiii). Singer was co-author of a paper in 2003 on the same subject, so definitely not a new subject, even if I have never heard of the word before!
Here are some other resources about syndemics:
Medical Anthropology Wiki.
A collection of articles on Co-Infection and Syndemics, from BioMed Central.
Syndemics Project, Florida International University.
Singer, Merrill, ed. (2009). Introduction to Syndemics. John Wiley & Sons.