Need more info. Don't know age. Toddler in daycare? No daycare but first time in school? Other issues? Little kids who get exposed to lots of little kids get sick a lot-exposed to lots of viruses. So do parents of little kids, and first time teachers of little kids. Adorable little snot factories. Wash hands, use purell. See md make sure no underlying issues, e.g. Ge reflux, ear, nose throat-more common.
How old. Age pending, it could be post nasal drip, allergic or inflammatory/ weather related.
Low immunity. Low immunity causes body to get sick - need adaptogen. Give garlic chopped up into tiny pieces with tsp of water daily - like a pill will kill bugs no need to crush garlic.