If you stressed your body too much, then you can get sick. This stress can be excessive exercise, other physical stress, injuries, internal stress, emotional stress, lack of sleep, high sugar intake, high toxic exposure (alcohol, cigarrete smoke, etc), lack of nutrients, lack of calories, antibiotic use, and the list goes on and on.
If you were too stresses, then your immune system will get run down. At this point any oportunistic disease can jump right in and take over easier than when you're healthy.
Either that or you just ate something really bad or something.