There are lots of things that can contribute to the red and swollen gum condition called gingivitis. Hormone changes, cancer, HIV, diabetes all may be contributors, but the primary cause is plaque. Control plaque and it's related bacteria, and you control gingivitis. Proper brushing and flossing is key, and your dentist can do thorough cleanings or prophylaxis. As with many other conditions, it helps to eat properly and if you smoke, give it up.