It happens. Since the beginning of the pandemic I've caught something exactly once, a couple of months ago. I did a PCR test however while symptomatic and it came out negative. So did my wife when she got it a couple of days later. My point is that something got through the defense protocol and could easily have been SARS-CoV-2. I only know it wasn't because I tested for it.

Cause masks prevent others from getting your diseases.

If everyone else wasn't wearing a mask, then you were prone to getting their diseases.