Recurring oral problems are very often caused by improper oral hygiene, and by being exposed to the same microbes and the same pathogens over and over, time and time again. This is because we eat the way we do, have oral flora that is the way it is, and encounter the same microbes from the air, from the people we kiss, from the foods we eat, as each ecosystem and climate has its own set of microbes and bacterial life that flourish on it. One of the places that microbial life flourishes is the toothbrush.
Although the toothbrush, that tool which you use to clean your teeth on a regular basis and is constantly slathered with antibacterial, high fluoride toothpastes is the last place you would look for bacteria that live in your mouth, it is actually one of the places that they thrive on and that lets them constantly come back to re-infect your oral cavity. The bristles catch the bacterial life and they can set up shop there, living a meek and miserable existence, but an existence nonetheless. Eventually the colonies become so strong that washing the toothbrush out and rinsing it in hot water, etc. will have no more effect, and you will reintroduce bacteria into the mouth.
What to do about it
The toothbrush is still your number one ally against tooth decay, and you must use it if you want to keep your tooth and avoid tooth decay. There is no way to circumvent it, so the best thing you can do is to rinse it very often, before and after use, possibly even rinse it with very hot water, as this will break up the biofilm and plaque on your toothbrush. The other thing to do is to change your toothbrush frequently, like once every 2 months or so. A good way to spot when a toothbrush needs to be disposed of is if it has tiny brown spots on the bristles, as they are the bacterial colonies themselves.