Time lag in disease models with recruitment

In modelling sexually transmitted diseases, it is standard to divide the po pulation into a very active core group and a larger but much less active no ncore group. Transitions between these two groups may be affected by the pr evalence of disease in the core group. There may be a time-lag in this effe ct, and we explore the relation between such a time-lag and the stability o f an endemic equilibrium. (C) 2000 Elsevier Science Ltd. All rights reserve d.