We dry it out further by heating it. If so, your house is too dry. Pets like a dog or cat are a good marker for dryness. If you get shocks petting Fido, your home is way too dry. The first line of defense against colds, viruses and bacterial infections is a healthy and intact nasal lining and upper airway lining.
Germs are mostly airborne or spread by germ-covered hands.
Though washing hands helps to avoid spreading germs, a breakdown in your nasal lining is a breakdown in your defense system. With dry air, the nasal lining dries out and develops tiny cracks that allow germs to penetrate. Humidification of your environment will help keep your nasal and respiratory linings healthy and moist. It's no coincidence that more viral and bacterial infections, such as colds, flu or bronchitis, occur in winter than summer. The lack of humidity has a lot to do with that.