In most cases they do no real harm to the plant. Large populations can cause wilting and poor plant growth due to minor root pruning. Unfortunately, pesticides used to control soil insects may be as harmful to the plant as the insects. House plant diseases are not seen as often as insects. Diseases such as powdery mildew or various root and stem rots are encountered, but can be controlled to some extent with proper plant care.
Most problems resembling diseases are the result of improper care.