My kids have had three colds this year! Normal?

My son is 2.5 and teething the past few months. My daughter is only 4 months and fully fed. She is so young to have 3 colds already but my son gets them and then slobbers all over her. My son is not usually sick often and never goes to daycare so not sure where he gets all these colds lately. Does teething make them get sick easier? Do younger siblings usually get sick a lot so young?