The rate of multiplication is related to available food and environmental conditions. Raising the temperature, although frequently recommended for parasite problems, has an adverse effect on combating bacterial infections. When tropical fish are sick we raise temperature to increase their metabolism and promote antibody production. If we do that on a bacterial disease it could be disasterous. In this race, bacteria will always win.
Bacteria are present in every place of the planet.
It has to be this way because their job is to recycle natural matter. When they find an appropriate environment, such as an aquarium, they start to multiply and feed. In a poorly maintained aquarium, bacteria grows rapidly which can contribute to oxygen-depletion which leads to fish stress. Stressed fish have a lower pathogen resistance. Discus in particular have a huge slime surface which could be attacked.