Table 1.

The inclusion of environmental processes in epidemiological models depends on the pathogen lifestyle (sapronotic versus parasitic) and relative contribution of different transmission pathways. Flow diagrams for epidemiological models based on these two criteria are presented. Hosts are categorized in two epidemiological states: susceptible (S) and infected (I). For simplicity, a recovered (R) is not represented. Pathogens in the environment are represented by the compartment P. Solid lines represent flows among variables and dashed arrows indicate when a compartment I or P contributes to new infections.

pathogen classification	examples	model structure
(a) sapronotic only:	Legionella pneumonia
reproduce in the environment	Mycobacterium ulcerans
environment-to-host transmission	Fusarium solani
(b) sapronotic and parasitic:	Vibrio cholera
reproduces in the environment/host	Bacillus anthracis
environment-to-host transmission	Geomyces destructans
other transmission modes
(c) parasitic only:	chronic wasting disease (prions)
reproduces in the host	Cryptosporidium parvum
environment-to-host transmission	Clostridioides difficile
other transmission modes	Salmonella enterica
	hantavirus
	influenza
(d) no environmental role:	Chlamydia trachomatis
reproduces in the host	human immunodeficiency virus
other transmission modes	Treponema pallidum