Gingivitis is an inflammation of the gum (image below shows gingivitis in a young girl). Gingivitis is due to bacteria in the dental plaque located at the gum margin and it will manifest in anyone leaving plaque to accumulate undisturbed, however, being almost asymptomatic, it is often overlooked. 

Despite plaque might be difficult to recognize to the naked eye, the use of a disclosing agent reveals abundant dental plaque accumulation (in purple). If seen in the microscope, the plaque-forming bacteria are clearly visible (dotted grey circle).

Gingivitis usually heals promptly upon establishing correct oral hygiene routine (photo below).

Some of the subtle changes in the gum occurring due to gingivitis are visible in the yellow circles in the top row image.

If you want to know how healthy your gums are please check the self-assessment table.