When a wisdom tooth is problematic, symptoms may include: pain and swelling of the gum overlying the impaction (due to either infection of the operculum or trauma from the tooth above hitting into it, or a combination of both). For example, swelling that arises from infection may make the upper tooth impinge onto the gum traumatizing it more and causing a vicious cycle.
The cause of these problems is that when a wisdom tooth is impacted, a flap of gum will lie over it. As it is difficult to clean effectively under the gum flap, bacteria will proliferate here and the gum will become inflamed.
An impacted tooth can also be present in the mouth without you even knowing about it, because it may not be causing any symptoms. However, other problems can also be associated with impacted wisdom teeth. They are prone to decay. This is because food can become trapped around them, and they are difficult to clean. The tooth lying beside will also be at increased risk of decay for the same reason. The tooth may become sensitive or painful.
Likewise, the area is more prone to gum disease because it is difficult to clean. Rarely, cysts, and other such growths, may form around an impacted tooth. Therefore, even when they aren't causing pain, or other noticeable symptoms, they should be checked regularly. Your dentist can make sure that all is well in the area, or if any damage begins to occur, then the situation can be remedied sooner rather than later.