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. Author manuscript; available in PMC: 2018 May 29.
Published in final edited form as: Lancet. 2017 Mar 4;389(10072):978–982. doi: 10.1016/S0140-6736(17)30403-8

Figure. Models of epidemics co-occurring with HIV involving syndemic synergy (left) and mutual causality (right).

Figure

In the syndemic model in the left panel, the two epidemics are not mutually causal but demonstrate synergy: epidemic A has a greater impact on the HIV epidemic in the presence of epidemic B, epidemic B has a greater impact on the HIV epidemic in the presence of epidemic A, and the combined effect of epidemics A and B on the HIV epidemic exceeds the sum of their independent effects were no synergy present. In the model of mutual causality in the right panel, the two epidemics do not demonstrate synergy but have reciprocal relationships with each other: epidemic A exacerbates epidemic B, which in turn worsens epidemic A, and both epidemics have similar bidirectional relationships with the HIV epidemic.