Thrush is naturally accruing so it will always be present. The hoof anatomy and genetics of the horse can cause and actual Thrush infection itself. When Thrush actually becomes an “infection” is when the the bacteria get trapped. This can be caused by poor trimming from the farrier, basically not cutting back extra tissues from around the frog and such allows crevices where the thrush can be safe and sound to multiply. Another factor is purely hoof genetics. If for example the horse has contracted heals, it has a poor ability for that area of the hoof to breathe. Air kills bacteria and if its locked in there, its a breading ground for bacteria and fungus. Again a farrier can help that situation as well.
Dry stalls do not always help thrush infections. Having overly dry feet can actually make it worse. If the thrush becomes “trapped” and started to encapsulate, what happens is that it turns into an incubator for it to grow and it gets tar like and started eating away the hoof and from that point you can get a much worse problem if it gets bad enough that it starts getting into the sensitive structures of the hoof.
Best things to keep from having thrush become a problem is to pick out the hooves very often like daily or more, proper trimming and shoeing and not letting the hooves get too overgrown. Using thrush treatment to help kill whats there is OK but you need to remember its a naturally occurring bacteria that has a purpose so if you use treatment too much it can damage the hoof tissues causing more problems.
If you use 50% bleach and water, DO NOT use turpentine on the hoof (can make pretty smoke, good luck catching the horse when your done) and do not get it on the hairline as it can damage hair follicles and leave white hairs permanently above the hooves.