As we exam Patricia's oral cavity we can see that her gingiva is red around the margins, swollen and when we probe or even touch them, they bleed easily. Gingivitis is primarily an inflammation of the gingival margins. It does not involve bone loss and is usually a result of bacterial plaque/biofilm remaining in and around the gingival margins and interproximal areas.
When the area is probed with the periodontal probe, there is usually no evidence of bone lose. The diagram shows a probe inserted into a sulcus around a premolar, it measures 4 mm (each black and silver band represent 3 mm increments). Three millimeters is a normal reading for this area. But because the marginal gingiva is erythemic (red) and edematous (swollen), the reading has increased by about 1 mm. The lack of bone loss can be confirmed when reviewing the radiographic images.