The answer to that is easy: Nothing. Well, how about not discarding the question straight away and have a little think about it?
Everything around us is energy, everything has a vibration. Positive emotions like love, joy, fun, gratefulness have higher vibrations and negative emotions like sadness, envy, jealousy have lower vibrations. Sickness also has a vibration, and I think you can guess it, it is not particularly high.
When you have an airplane full of people, and somebody is sick, those germs are nicely distributed to the whole of the plane through the ventilation system. Will every single passenger get sick on that plane? No, the ones with higher immunity, alias higher vibration will not get sick. The virus would only be able to attach to those with a lesser vibration than it's own level. So when our vibration is lower we are more prone to sickness. What could be the reason why our vibration is lowered?
My theory is, that we get sick to gain something from having the cold or flu. What can we gain? Several things actually:
1. You are tired, exhausted but won't take a rest - you need time off - with sickness you are forced to get it
2. You are afraid to face something - you want time away - with sickness you have an excuse, nobody will expect you to go in like this (hello sick child night before an exam)
3. You need love and attention - you need to connect - with being sick maybe someone will finally look after you, notice you and bring you some tea or TLC or just even negative attention, noticing you are there
4. A good old spiritual cleanse or transition is happening - you are growing - just getting rid of the old, making space for the new
Now if you are sick often, just take a quick look at these and see if any of them resonate with you and if you could give these to yourself without being forced into it, as in getting sick. If none makes sense, and you think it is 4. regularly than you must be enlightened by now :-).
Would like to hear your thoughts on these.