We study a deterministic SIS model, which takes into account seasonal variations as well as heterogeneities in the population. The appropiaate threshold condition is found (the condition for the disease to remain endemic). © 1980.
Aronsson, G., & Mellander, I. (1980). A deterministic model in biomathematics. asymptotic behavior and threshold conditions. Mathematical Biosciences, 49(3–4), 207–222. https://doi.org/10.1016/0025-5564(80)90079-6