That is to say that they carry the various different types of bacteria, virus on them which spreads the disease they bring with it. Stressed Plants Lack of nutrition or improperly feed plants and soil will cause stress which in turn attracts the diseases and pests. All chemical fertilizers kill off the beneficial bacteria in the soil as well as kill off beneficial hairs on the root systems of plants. It is these fine hairs which house the bacteria needed for root system to absorb nutrition. Over watering will cause a great deal of stress to both plants and soil.
City water has various chemicals added to it to kill off the bacteria, like fluoride.
This causes the soil to become sterilized over a period of time. Reduce stress, reduce stress, reduce stress. Not only the affected plants but also yourself. Quite often we are so stressed out that our immediate environment also gets stressed out. Reduce stress of the affected plant or area or tree.