Gum disease starts with bacteria buildup. If you are not properly brushing and flossing, plaque will accumulate on your teeth. Within 2 weeks, plaque is mineralized into calculus that cannot be removed with brushing and flossing. Plaque and calculus both cause bad breath. Dental cleanings are necessary to remove calculus.
Plaque and calculus are sources of bacteria that irritate your gums (inflammation). The result is sensitive, red, puffy gums that bleed easily (especially when brushing/flossing). If your gums stay inflamed for a long period of time, you will permanently lose bone around your teeth.
As you continue to lose bone around your teeth, your teeth become loose. If you lose too much bone, you will end up losing those teeth. Gum disease is the number one cause of tooth loss in adults. Bone loss and tooth loss is not a natural result of aging; it is entirely preventable!
Here you can see a 42 year old patient with severe gum disease and bone loss:
Most or all of these teeth will need to be removed.
Compare that to this 84 year old patient:
This 84 year old patient has some slight bone loss, but is doing great!
If a tooth needs to be removed because of gum disease, waiting too long will limit your options for replacement. Here are 3 teeth that should have been removed long ago: