Fig. 1: Understanding the four epidemiologic stages of IBD evolution.

A guitar-string analogy demonstrates the four epidemiologic stages of inflammatory bowel disease (IBD) evolution, in which incidence is represented by the vibration of the guitar string and prevalence would be the pitch we hear. In stage 1 (emergence), the guitar string is slack; few vibrations pass down the neck. Low-frequency waves symbolize the rare, sporadic appearance of IBD cases, and the ear hears only a faint, low pitch (prevalence is minimal). In stage 2 (acceleration in incidence), environmental forces tighten the tuning peg, raising string tension. Wave frequency climbs, representing the accelerating influx of new cases. The perceived pitch rises, and prevalence begins its upward trajectory. In stage 3 (compounding prevalence), the string is now at maximal tension. Wave frequency plateaus at its highest level with the continuous stream of vibrations causing more crests to fill the neck of the guitar. Prevalence keeps climbing even though incidence is no longer increasing. In stage 4 (prevalence equilibrium), with no further tightening, the propagating wave gradually loses amplitude, illustrating how the annual addition of new cases is offset by the mortality of an ageing IBD population. The pitched sound stabilizes and softens, mirroring a steady-state prevalence.