Right about this time of year is when the gate-crashing usually starts. My 4-year-old and 8-year-old bring the uninvited guests home from school. First sniffles, then coughs, and finally all-night hacking. The cycle repeats itself through the winter on a seemingly endless loop of crusty noses, balled-up tissue paper, and general grumpiness. Whoever discovers a cure for the common cold will be