Periodontal Disease happens if gum inflammation (red/swollen gums) continues over time. If not brushing and flossing and leaving plaque and tartar present this starts to destroy the gum tissue. When it starts to destroy the gum tissue it eventually starts to destroy the bone around the tooth. And once bone is lost around the tooth it doesn't come back. This is why it is so important to brush and floss and keep teeth healthy.
Not only does the inflammation in the mouth affect your teeth but it can affect the rest of you body. This can affect diabetes, the heart (such as high blood pressure), Alzheimer's disease, obesity, osteoporosis, along with other diseases. You need your teeth for the rest of your life. If you don't take care of them now, it can cost a lot of money from hundreds to thousands of dollars to maintain a mouth that has been affected by periodontal disease. If you have periodontal disease you typically need to start with having scaling and root planning to help the gums heal around the teeth and then you need to maintain it by seeing the dentist every three (3) months instead of every six (6) months. Six (6) months is only for a person that has a healthy mouth!