Joli, Wang, Weiss et al, [4] in 2009 Modified Codegc/s model by a threshold
pathogen density for infection, with a careful discussion on human-environment
contact and in-reservoir pathogen dynamics. They called their model as z'SIR model
where “i” represents for indirect transmission dynamics from contact with reservoirs
containing human pathogens but not from human to human directly as shown below
as state B. They assumed that there is the explicit incorporation of a minimum