How do people get gingivitis?

Not brushing: If you do not brush and floss you get build up of plaque and calculus on you teeth this cause you gingiva to become inflamed. This is gingivitis, if untreated it become periodontitis that's when it progresses from the gingiva and effects the supporting bone around the teeth. Your teeth become loose and you can lose them. Do oral hygiene and visiting you dentist every six months is important.

Like this:: Bacteria are normal inhabitants of the mouth living in a thin film called plaque. Brushing and flossing get rid of plaque. Plaque not removed can harden & form “tartar”, a hard mineral shell, that brushing can’t remove. Only a professional cleaning by a dentist can remove tartar. When plaque builds on the tartar surface, it irritates and erodes healthy gum tissue. This is called gingivitis.

Yes: True gingivitis, or inflammed gums, can many times be improved just from better brushing and flossing, with emphasis on the latter. Most people don't floss, and that is a main cause of gingivitis. And it can begin in as little as six days without flossing. The cause is the oral bacteria, getting under the gum and your body's reaction to that inflammation.