A patient with streptococcal sore throat takes penicillin for two days of a prescribed 10-day regimen. Because he feels better, he then saves the remaining penicillin for some other time. After three more days, he suffers a relapse of the sore throat. Discuss the probable cause of the relapse.

I have always been told that it’s important to finish your antibiotic even if you think your symptoms have gone away. But I don’t know the exact reason behind this.