When Two Epidemics Overlap: Modeling Malaria–Lassa Fever Co-Infection Dynamics

Across West Africa, malaria and Lassa fever co-circulate aggressively, yet public health strategies routinely treat them in isolation. This work presents a population-level mechanistic model to explore their coupled dynamics. Utilizing a 16-compartment deterministic framework, global sensitivity analysis, and invasion reproduction number theory, we examine whether the presence of one endemic disease alters the persistence and invasion threshold of the other. Our findings show that coexistence is the structural default; because malaria and Lassa fever operate through independent transmission cycles, they stably co-circulate without competitive exclusion. However, mathematical simulations demonstrate that co-infection introduces a severe epidemiological synergy, amplifying the overall disease burden and generating up to a 25% increase in cumulative mortality compared to either disease acting alone. By identifying the specific transmission pathways driving this excess mortality, the model provides actionable insights for regional public health planning. The analysis indicates that reducing the co-infection burden does not require joint interventions; instead, strategically targeting whichever disease is more locally treatable can disproportionately suppress excess mortality in co-endemic regions.