I have a tree (shrub) that seems to have a pest or disease.
Insects are part of the ecology of plants and most plants can easily tolerate an infestation of insects. Occasionally, a new pest outbreak will seem overwhelming but natural predators tend to show up and ultimately reduce the "pest" to a minor occurrence. Such was the case for Gypsy moth and Viburnum Leaf beetle.
Diseases tend to not be able to kill a plant unless the plant is weakened (compromised) by other stresses such as drought, improper soil match or incorrect light level.
Pest and plant questions are readily handled by the University of Guelph's Lab Services. This is a fee-for-service organization but they are equipped with current information on pests and diseases and choices in control - including "do nothing" if nothing needs be done.