All entities are parasites that drain life force from their host.
This can result in a variety of symptoms revolving around fatigue and loss of pep. This drop in vitality acts as a facilitating factor for nearly any disease. Apart from their general effect, entities often have local effects. This can result in a great variety of symptoms, from headaches and pre-menstrual syndrome to much more severe problems such as miscarriage or even tumours. It would be ridiculous to try to make entities scapegoats for all human problems.