How Do They Do It?

I have six kids. At one point, I had a 4 yr. old, a 3 yr. old, a 2 yr. old, and a newborn. They were normal kids...got sick just like anyone else that age. Sometimes they gave each other illnesses over and over. Colds...runny nose, headache...just never seemed to bother them. That ol' nose would run and run, and they would keep on playing and running and jumping. I have had a head cold for a few days, and this runny nose thing is about to kill me! I keep a paper towel with me (not so soft, but surely is absorbent!)I was bending over to mop the bathroom and I thought my head would explode. My kids never stopped... snot pouring down their faces, heads- I am sure- pounding in pain! How did they do it?