Sure, I suppose it could just be a string of bad luck. Maybe one was a cold, one was a mild touch of flu, and perhaps you could throw in a slight allergy to something. At some point you will have antibodies for every known contagion. You will be bullet proof, as they say.
But just to be safe, make sure you're eating a healthy diet, drink plenty of water, avoid stress and get a good nights sleep. Don't take any chances with your health and maybe you can strengthen your immune system.